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Exception. Modified Cadence.'-Exercise 1. "This monument is a plain shaft. It bears no inscription, fronting the rising sun, from which the future antiquarian shall wipe the dúst. Nor does the rising sun cause tones of music to issue from its súmmit. But at the rising of the sun, and at the setting of the sun, in the blaze of noon-day, and beneath the milder effulgence of lunar light, it speaks, it acts, to the full comprehension of every British mind, and the awakening of glowing enthusiasm in every British heart."

2. "I speak not to you, sir, of your own outcast condí ion. -You perhaps delight in the perils of martyrdom. I speak not to those around me, who, in their persons, their substance, and their families, have endured the torture, poverty, and irremediable dishónour. They may be meek and hallowed men,-willing to endure."

3. "The foundation on which may seem to you deep and firm. the howling blast, and the beating

treacherous sànd."

you have built your hopes,
But the swelling flood, and
rain, will prove it to be but

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weeds of luxury will spring up amid the flowers of art." Exercise 1. "In a rich soil, and under a soft climate, the

2. "All the wise institutions of the lawgiver, all the docslum-trines of the sage, all the ennobling strains of the poet, had perished in the ear, like a dream related, if letters had not preserved them."

RULE III. 'Moderate' Falling Inflection, of complete sense.Exercise 1. "Animal existence is made up of action and ber: nature has provided a season for each."

2. "Two points are manifest: first, that the animal frame requires sleep; secondly, that night brings with it a silence, and a cessation of activity, which allow of sleep being taken without interruption, and without loss."

3. "Joy is too brilliant a thing to be confined within our own bosoms: it burnishes all nature, and, with its vivid colouring, gives a kind of factitious life to objects without

sense or motion."

4. "When men are wanting, we address the ànimal creation; and, rather than have none to partake our feelings, we find sentiment in the music of birds, the hum of insects, an:l the lowing of kine; nay, we call on rocks and streams and forests

to witness and share our emotions."

5. "I have done my duty:-I stand acquitted to my conscience and my country: -I have opposed this measure throughout; and I now protest against it, as harsh, opprèssive, un àlled for, unjust,—as establishing an infimous precedent, by retaliating crime against crime,- -as tyrannous,— cruelly and vindictively tyrannous."

Exception. Plaintive Expression.'

Exercise 1. "I see the cloud and the tempest near,

The voice of the troubled tide I hear;

The torrent of sorrow, the sea of grief,
The rushing waves of a wretched life."

2. "No deep-mouthed hound the hunter's haunt betrayed,

No lights upon the shore or waters played,

No loud laugh broke upon the silent air,

To tell the wanderers man was nestling there."

3. "The dead leaves strew the forest walk,
And withered are the pale wild flowers;
The frost hangs blackening on the stalk,
The dew-drops fall in frozen showers :-
Gone are the spring's green sprouting bowers.
Gone summer's rich and mantling vines;

And Autumn, with her yellow hours,
On hill and plain no longer shines."

8. "The dimensions and distances of the planets, the causes of their revolutions, the path of comets, and the ebbing and flowing of tides, are now understood and explained."

4. "The mighty pyramid, half buried in the sands of Africa, has nothing to bring down and report to us, but the power of kings, and the servitude of the people. If asked for its moral object, its admonition, its sentiment, its instruction to mankind, or any high end in its erection, it is silent;-silent as the millions which lie in the dust at its base, and in the catacombs which surround it."

5. "Yes, let me be frèe;† let me go and come at my own will; let me do business, and make journeys, without a vexatious police or insolent soldiery to watch my stèps; let me but that which is set by the common wèal; subject to no law think, and do, and speak, what I please, subject to no limit but that which conscience binds upon me; and I will bless my country, and love its most rugged rocks, and its most barren soil."

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Exception 3. Poetic and Pathetic Series.'
Ex. 1. "Wheresoe'er thy lot command,
Brother, pilgrim, stranger,

God is ever near at hand,

Golden shield from danger."

2. "Rocks of granite, gates of brass,
Alps to heaven soaring,

Bow, to let the wishes pass

Of a soul imploring."

3. "From the phantoms of the night,

Dreaming horror, pale affright,

Thoughts which rack the slumbering breast,
Fears which haunt the realm of rest,

And the wounded mind's remorse,

And the tempter's secret force,

Hide us 'neath Thy mercy's shade."

4. "From the stars of heaven, and the flowers of earth,
From the pageant of power, and the voice of mirth,
From the mist of the morn on the mountain's brow,
From childhood's song, and affection's vow;

From all save that o'er which sóul bears sway,
There breathes but one record,-'passing away!'"

4. "What is human life, but a waking dream,-a long reverie, in which we walk as in a vain show, and disquiet ourselves for naught?' In childhood we are surrounded by a dim, unconscious present, in which all palpable realities seem for ever to elude our grasp; in youth, we are but gazing into the far future of that life for which we are consciously pre-being, like enumeration, cumulative in effect, and corresponding, thereparing; in manhood, we are lost in ceaseless activity and enterprise, and already looking forward to a season of quiet and repose, in which we are to find ourselves, and listen to a voice within; and in old age, we are dwelling on the shadows of the past, and gilding them with the evanescent glow which emanates from the setting sun of life."

Falling slide of contrast to the preceding clause.

• All emphatic series, even in suppositive and conditional expression, fore, to climax in style, are properly read with a prevailing downward slide in the suspensive' or slight form, which bel nas to incomplete but energetic expression, and avoids, accordingly, the low inflection of cadence at a period.

slide for the slight suspensive one. But the tone, in such cases, will stil
Emphasis, and length of clause, may substitute the moderate' falling
be perfectly free from the descent of a cadence, which belongs only to the
period.
The inflection of any clause always lies on the emphatic word; and, if
that word is a polysyllable, on the accented syllable chiefly, although not
always exclusively

5. "When the summer exhibits the whole force of active nature, and shines in full beauty and splendour; when the succeeding season offers its 'purple stores and golden grain,' or displays its blended and softened tints; when the winter puts on its sullen aspect, and brings stillness and repose, affording a respite from the labours which have occupied the preceding months, inviting us to reflection, and compensating for the want of attractions abroad, by fireside delights and home-felt joys; in all this interchange and variety, we find reason to acknowledge the wise and benevolent care of the God of seasons."

HISTORICAL MS. MAGAZINE.

SIR, I was very much pleased with the letter of "T. J." pub. lished in No. 105 of the P. E., and think his plans excellent. This note suggested to me the idea of starting a manuscript Magazine on the subject of HISTORY, commencing with the history of our own dear iste. Perhaps, sir, you will be so obliging as to publish this letter in the P. E., as I wish it to meet the eyes of several young persons who are desirous of studying the science of history in the manner proposed. Any person wishing to join this magazine is requested to communicate with the editor by letter, directed, " 'Sigma, Post-office, 103, Tottenham-court-road, London." Before I conclude, I must thank you for your generous and disinterested labours for the education of your countrymen; and wishing you every success I remain your constant reader, Σίγμα.

SELF-EDUCATION.

6. In that solemn hour, when exhausted nature can no longer sustain itself, when the light of the eye is waxing dim, when the pulse of life is becoming low and faint, when the breath labours, and the tongue falters, when the shadow of death is falling on all outward things, and darkness is begin- SIR,-Fourteen months ago a friend placed in my hands a numning to gather over the faces of the loved ones who are weep-ber of the P. E. I had a great desire to improve myself, so I ing by his bedside, a ray of immortal Hope is beaming from his features: it is a Christian who is expiring." Note 2.-Exercise 1. " Repeated and Heightening Rising Inflection.' I ask, will you in silence permit this invasion of your rights, at once wanton, mischievous, uncalled for, and unnécessary? Will you patiently tolerate the annihilation of all freedom,-the appointment of a supreme dictator, who may, at his will, suspend all your rights, liberties, and privileges? Will you, without a murmur of dissent, submit to a tyranny which nearly equals that of the Russian autocrat, and is second to that of Bónaparte ?"

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2. Repeated and increasing Falling Inflection.' + "Was it the winter's storm, beating upon the houseless heads of women and children; was it hard labour and spare mèals;was it disease,— -was it the tomahawk; was it the deep inalady of a blighted hope, a ruined énterprise, and a broken heart;was it some, or all of these united, that hurried this forsaken company to their melancholy fate?"

3. "Yes, after he has destroyed my belief in the superintending providence of Gòd,—after he has taught me that the prospect of an hereafter is but the baseless fabric of a vision, -after he has bred and nourished in me a contempt for that sacred volume which alone throws light over this benighted world, after having argued me out of my faith by his sophistries, or laughed me out of it by his ridicule,-after having thus wrung from my soul every drop of consolation, and dried up my very spirit within me ;-yes, after having accomplished this in the season of my health and my prosperity, the sceptic would come to me while I mourn, and treat me like a drivelling idiot, whom he may sport with, because he has ruined me, and to whom, in the plenitude of his compassion,-too late, and too unavailing,-he may talk of truths in which he himself does not believe, and which he has long exhorted me, and has at last persuaded me, to cast away as the dreams and delusions of human folly."

CORRESPONDENCE.

THE "WORKING MAN'S FRIEND" AND P. E. SIR, I have, until this day, been deprived of the advantage of possessing your valuable "EDUCATOR;" but having now procured the four volumes already published, and cursorily inspected them, I cannot refrain from expressing my delight and astonishment at the valuable and extensive information therein contained. Doubtless you have experienced the loss of a "friend," and can therefore imagine with what regret I parted from my late companion and "instructor" ("The Working Man's Friend"), than which a more useful work for the operative classes never appeared.

I was promised a valuable substitute in the "P. E.," and I doubt not that my disappointment at losing my old "Friend" will be amply compensated by cultivating the acquaintance of the new. With sincere repect, I am, sir, yours respectfully, Tenterden, April 27th, 1854.

HENRY DRIVER.

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bought all the back numbers; and I have since continued to take in the monthly parts regularly. I first began my studies with Dr. Beard's Lessons in English, the Lessons in Arithmetic, and Mr. Bell's Lessons in Phonetic Short-hand. I feel satisfied with the progress I have made; but I have no person to guide or instruct me in my endeavours to improve myself. I am a married man, 29 years of age, and a policeman on a railway; part of my time is spent on night duty, and it is when I have been walking to and fro on my dreary post, that I have committed to memory the principal portion of Dr. Beard's Greek and Latin stems.

I feel anxious to commence the study of the Latin Lessons; but, before doing so, I wish to have your opinion of my hand-writing and the composition of this letter. I know my hand-writing is capable of improvement; but since I have taken in the P. E. it is improved very much, from practice in writing out the exercises. Lessons; but do you not think it will be sufficient if I go through I can hardly yet venture to discharge myself from the English them again with the Latin Lessons? I can write Mr. Bell's system of short-hand, and can read it afterwards; and I hope soon to be able to report a sermon, when I go to a place of worship, which I don't do very often, as I only get one half Sunday in four, having to be on duty all the rest of the time. If Mr. Bell has published his sys'em, or vocabulary of reporting logograms, I should like to procure them. With grateful thanks for the benefit I have received from the P. E., I remain your obedient servant, G. C. (Leamington.)

[In answer to this interesting letter, an example to many, we say that his hand-writing does him great credit, although capable of improvement; that it will be sufficient to go through the English Mr. Bell's Phonetic Short-hand is truly praiseworthy; and that Mr. Lessons again along with the Latin; that his perseverance with Bell has not yet published his Vocabulary of Logograms.]

ANSWERS TO CORRESPONDENTS.

E. J. BREMNER (Carlisle): Received the solution of 50 of the Centenary of Problems; very well done.-ANGLO-SAXON (Longport): Thanks for his broad hints.-DUTCHMAN: We should be glad to oblige him.-B. ß. (Dublin): Certainly.-Q. PRINGLE (Glasgow): Received.-J. STEELE (Mauchester): We don't know any good Italian grammar. We believe that the ancient Greek would be some help to the modern, but how much we can't say. A modern Greek Testament can be had of the British and Foreign Bible Society; and by inquiry there, information may be had about modern Greek grammar.-T. W. (Leeds): The French Dictionary is com pleted; see our Literary Notices.

to curing bad spelling in any other way than by consulting a dictionary or J. G. F. R.: The map of European Russia was published in No. 102. A books, we know it not; and we would scarcely recommend to his trial the printer's advice to his devil, viz. to boil a small copy of Johnson's Dictionary in milk, and eat it over night for his supper, in the hope that he would be able to spell next morning.

W. R. TURNER: Thanks for his hints; some of the corrections have been

made.-M. J. DE COGAN (Liverpool): Inquire no more about the party.
the sum of two numbers is not their product.
THE ANCHORITE (Roehampton): His discovery is very like a mare's nest;

W. S.: Three per Cent. Stock means property in the public funds which yields 3 per cent. per annum to its possessor.-W. H. T. (Hackney): The Lessons on Phonetic Short-hand began in No. 40.-J. DOWELL (Birmingham): The spire question might be answered if the data were more complete; the nature of the cavity s not stated, whether it is also pyramidal or otherwise.-H. R. (Jewin-crescent) will be kept in view.-J. AILD (Macclesfield): We really cannot tell.-POOR SCHOLAR (Glasgow): See vol iv. p. 375 col. 2, line 27.-J. F. R. SMITH: 1. No. 2. No. 3. If a degree can be bought at any of the German universities, the more's the shame. 4. See p. 163, vol. ii.-TAU (York) asks too much; he may answer all his questions himself by studying our Lessons in Geometry in the P. E., and Cassell's Euclid-JOHN THOM, Secretary to the "Cassell's Popular Discussion Class" (but where he does not say), is informed, in answer to the question brought before the class, that, in the opinion of the Editor, if any one is

ERRATA.

DAVID: Vol ii. p. 327, col. 2, line 10, for D E read D F.

desirous of becoming a civil engineer or a chemist, we would recommend is of very litte moment.-S. A. G. (Bishop's Stortford): We may point out him first to apply his mind to the Lessons in the P. E. on Mathematics, a few errors in his poetry-wiled for whiled; gems for flowers; and sunChemistry, Natural Philosophy, Logarithms, and branches connected with shining for heaven's bliss. We have heard a proverb to the effect indicated these, as well as to "Cassell's Arithmetic and Algebra," before he reads the by the later phraseology, but we thought it sounded profane.-T. C. and books published by the first men of the day.-H. J. COCKBURN (Dews- J. C. (Bedforu): If Cleworth be the same as the Welsh Glewarth, it means bury): The petition was received and will be presented.-JAMES NEVILLE: a bold bear. If Cripps was originally applied to a curley-headed person, it The greatest common measure of the fraction in question 10, p. 373, vol. iii., may have come from Crisp; and if to a cunning hidden character, from is 3x-4x+5; and of that in question 11, is 4x2-9x+6; but the operation Crypt. Johnson says that Gar, in Saxon, means a Weapon; of course Garrequires the use of large numbers.-G. G. (Brechin): 21858 is right. side is Weaponside, or the man ready to fight with his weapon by his side.MARK ANTONY (Woolwich): A very good solution of the Problem J. G. (Manchester) should consult Hutton's Mensuration, where the difNo. 108, but not without the rule for Quadraticз.-J. TIMMS (Chesterfield):ferent forms of casks are explained; and there are practical books on The same may be said of his solution of the same problem. His solution gauging. of the Four-Ball Question under consideration.-E. J. BODEN (Kingsland) very properly calls our attention to the fact that there is a "Deb ting Club" held at the "Lamb Tavern Assembly Rooms," and fearing lest our correspondent, W. Beck, may stumble on this club, instead of a proper Literary Debating Society, warns him of its danger in the following words:-" In consequence of this club being held where it is, it is to be doubted whether few, or any, of those who may attend its meetings will abstain from the stimulating liquors' that are there to be had. I think you will agree with me, Mr. Editor, when I say that stimulating liquors' are not required in 'debating clubs. When we see that a bigotted attachment to a party or a creed too frequently engenders a malignant feeling, and a bitterness of heart completely alien to the calm spirit in which truth should be investigated, it becomes necessary to warn every one as to the probable consequences of a person, with beer or spirits in his head, essaying to take a part in the discussion of any question whatsoever." We certainly agree with the objec tions of our correspondent in reference to all "debating clubs" held at taverns, and we would strongly warn our readers against joining any such society; to this warning we may add another reason, taken from some homely verses just sent us by JOHN BEWLEY (Lanrigg), entitled the "Temperate Man's Admonition to the Drunkard:"

"You waste your precious hours of time,
Amongst the lowest grade;

Whilst you o'erlook your purse, your health,
Your family and trade."

W., who says he is ours respectively instead of respectfully, has not solved the Quadratic Equations without the rule for Quadratics, which was required.-J. P. HEATER (Crag): We rejoice in his progress in Algebra: his solution of the Problems 40 and 52 are right. The Lessons in English in the P. E, combined with the careful study of good writers, will enable him to master his own language. He should make himself better acquainted with English before he begins Greek. He should do all he can to master one lesson before he begins another.-AN O. s. (0-y): We cannot promise Dutch so soon as he would like.-AN IGNORANT YOUTH should begin with the "Lessons in English," in the P. E., and with the "Lessons in Arithmetic" in the same.-A. BOYD (Glasgow) has not solved the Equation as required. We do not see why the third vol., 1d. edition, will not bind with the fourth vol. We find that the proposed exchange can't be effected.— IGNO-UMPIRE (Pontnewynydd): We think Bristol lies the highest; but we have no table at hand to refer to.-A. W. A. P. (Chelmsford): We can't tell. --A SUBSCRIBER (Wells, Norfolk): Analysis of 100 parts of cow's milk— water 874 parts; butter 40; sugar of milk (lactin) and soluble salts 5:0; casein, albumen, and insoluble salts 3-6; the total being 100.-B. CRABTREE (Lambeth): As soon as we can lay hands on "the way that the water comes down at Lodore," we shall insert it; but we forget where we saw the whole of it.-AUTODIDACTOS: Only two or three of the numbers of the P. E. in his list are out of print; but they will be in print very soon, He should try again, as we do not wish him to be at the expense of postage. His question we insert:-"On the 5th November last I took my two boys to see the large fire usually made here (Knottingley) in commemoration of the Gunpowder-plot. The elder stood at the distance of 5 and 2-3rds feet, and the younger at the distance of 9 and 3-5ths feet from the fire; how much more heat radiated to the elder than to the younger boy ?"

EAST LOTHIAN PLOUGHMAN: The Latin poem received.-A. DONALD R. (Woolwich): The higher powers of a number are readily obtained without all the intermediate ones, by multiplying some of the earlier powers by each other. Thus, to find the 31st power of 2, or any other number; find first the cube; multiply this cube by itself, and it gives the 6th power; this multiplied by itself gives the 12th power; this again multiplied by itself gives the 24th power; this multiplied by the 6th power gives the 30th power; and this multiplied by the root gives the 31st power-DICK (Pembroke): To inscribe a square in a semicircle, draw a straight line equal to the diameter perpendicular to one of its extremities; and join the remote extremity of this perpendicular to the centre by a straight line, and from the point where this straight line cuts the circumference, draw a perpen dicular to the diameter, and it will be the side of the square required.LABORE VINCO: His solutions are correct.-AGRICOLA (Newark) will see by our solution of the Four-Ball Question, that his own is different and his answer not the same.-J. K. (Belfast): No. 101 P. E. is not out of print, and may be had on proper application to our agents.-AUTODIDACTOS (Knottingley proposes the following ancient question to some of our students, after having solved it himself:—

"When first the marriage knot was tied,

Between my wife and me,

My age to hers was just allied

As three times three to three;

But after ten and half ten years

We man and wife had been,

Her age to mine was, it appears,

As eight is to sixteen.

Now can you, learned students, say
Our ages on our wedding day?"

A. L. C. His solution is very good, but he will see one inserted as required. Thanks for his kind wishes.-J. M. (Coverdale): Very well; but see the one inserted.-DAVID: You are right; many thanks.-G. H. H. (Haslingden) asks our intelligent correspondents the following question: If it be 10 o'clock on Saturday night here, what will be the time at the antipodes, and what day of the week?"-J.S. BROOK (Leeds): The mistake

J. Wardle (Dean Mill): Vol. iv. p. 329, line 32 from top, for the whole read one-half of the whole.

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This work is intended to supply the people with such information relating to the study of the Bible as the POPULAR EDUCATOR has given in reference to Secular Instruction. It contains a Literary History of the Sacred Books -Accounts of their Original Text-Canonical Authority, and most Ancient Versions-The Principle and Laws of Interpretations, and the Methods of Discovering the Literal or Symbolical Meaning of Inspired WritingsIllustrations of the Geography and Natural History of Palestine-The Manners and Customs, the Laws and Worship of its People-The Antiquities of the Four Great Monarchies-The Fulfilment of Prophecy concerning them and other ancient nations-and the Fruits of modern Travel and Discovery in the East, etc. The work is written in a popular style, and is therefore specially adapted to supply Families, Sunday-school Teachers, and others, with that amount of information respecting the Holy Bible which they need in order to meet the charges of Infidels and the subtleties of Romanists, and to confirm and establish their own minds in the genuineness and authenticity of Holy Writ. Wherever the subject requires Pictorial Illustrations they are introduced.

ON PHYSICS, OR NATURAL PHILOSOPHY.

No. XXXVI.

(Continued from page 129.)

Vapours.-Aeriform fluids which arise from liquids by the absorption of caloric are called vapours; such as those produced from ether, alcohol, water, and mercury. Those liquids which possess the power of passing into the aeriform state are called volatile; and those which give out no vapour at any temperature are called fixed, as the fat oils. There are some solids, as ice, arsenic, camphor, and generally odorific substances, which give out vapours at once without passing through the liquid state. Vapours, like gases, are usually transparent and colourless; there are only a few coloured liquids whose vapours are also coloured.

Vaporisation. The passage of a body from the liquid to the vaporous state is known under the general name of vaporisation; but by this term is particularly understood the slow production of vapour at the surface of a liquid; and by ebullition, a rapid production of vapour in the mass itself. The latter is produced, under the ordinary pressure of the atmosphere, in the same manner as fusion, at a determinate temperature for each liquid. In the case of evaporation, the effects are different; for this process goes on at various temperatures in the same liquid. Yet beyond a certain point of refrigeration, all vaporisation appears to cease. Mercury, for example, gives out no vapour below -10° Centigrade; and sulphuric acid none below 30° Centigrade.

Elastic Force of Vapours.-Like gases, vapours have an elastic force, in consequence of which they act with a certain degree of pressure on the sides of the vessels which contain them. To prove the tension of vapours, and at the same time to render them sensible to the eye, a glass tube of siphon shape inverted is half filled with mercury, fig. 186, then a drop of

Fig. 186.

ether is passed into the shorter branch, which is closed, and the tube is then immersed in a water-bath about 45° Centigrade. The mercury will now sink in the smaller branch, the space A B will be filled with a gas having entirely the appearance of air, and whose elastic force evidently balances the weight of the column of mercury CD, as well as the pressure of the atmosphere which acts on D; this gas is the vapour of ether. If we cool the water in the vessel, or if we withdraw the tube from it, which will produce the same effect, the vapour which filled the space AB will rapidly disappear, and the drop of ether will reappear. If, on the contrary, the waterbath be heated more and more, the level of the mercury will sink below the point B, and thus the tension will be increased,

VOL. V.

Formation of Vapour's in a Vacuum.-In the preceding experiment, the passage of the liquid into the state of vapour takes place slowly. The same thing happens also when a volatile liquid is freely exposed to the air. In both cases the atmospheric pressure is an obstacle to the vaporisation; but it is no longer so when the liquids are placed in a vacuum. The elastic force of vapours then meeting with no resistance, their formation is instantaneous. To show this, several barometric tubes are placed in the same cistern, fig. 187. These tubes

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being filled with mercury, one of them, the tube A for instance, is employed as a barometer; then drops of water, alcohol, and ether, are introduced into the tubes B, D, and E respectively. It is observed that at the moment when the liquid enters the barometric vacuum in each of the tubes, the level of the mercury sinks, as shown in the figure. Now, it is not the weight of the liquid introduced which depresses the mercury; for this weight is only a very small fraction of that of the mercury displaced. There is therefore, in the case of each liquid, an instantaneous production of vapour, of which the elastic force acts upon the mercurial column. From this experiment it is also evident that the depression of the mercury is not the same in the three tubes; it is greater in the tube where the alcohol is, than in that where the water is; and greater in the tube where the ether is, than in either of the other two. We are thus enabled to state the following laws on the formation of vapours:

1st. In a vacuum, all volatile liquids vaporise instantaneously.

2nd. At the same temperature, the vapours of different liquids do not possess the same elastic force.

As an example of the second law, the tension of the vapour of ether is nearly twenty-three times greater than that of the vapour of water.

Maximum of Tension.-When a very small quantity of a volatile liquid, such as ether, is introduced into a barometric tube, it vaporises instantaneously and completely, and the column of mercury does not experience all the depression of which it is capable; for if another small quantity of ether be introduced, the depression will increase. By continuing this operation, a moment will at last arrive when the ether introduced into the tube will cease to vaporise and will remain in the liquid state. There is therefore, for a given temperature, a limit to the quantity of vapour which can be formed in a given space. In this case, the given space is said to be saturated. Moreover, at the instant when the vaporisation of the ether ceases, the depression of the mercury ceases. There

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is, therefore, a limit to the tension of the vapour, which varies with the temperature, but which, for a given temperature, is independent of pressure.

In order to show that, in a closed space saturated with vapour and containing liquid in excess, the temperature being constant, there is a maximum of tension which the vapour cannot pass, whatever may be the pressure, we employ a barometric tube immersed in a deep cistern, fig. 81, p. 277, vol. iv, This tube is first filled with mercury, and a quantity of ether is passed into the tube sufficient to saturate the barometric chamber; there is then some liquid in excess, and the height of the mercury in the tube is ascertained by means of a scale fixed to the cistern. Now, whether the tube be immersed to a greater depth, which tends to compress the vapour; or whether it be raised, which tends to expand it, the height of the mercurial column remains constant. The tension of the vapour, therefore, remains the same in both cases, since the depression neither increases nor diminishes it. Hence, it follows, that when the vapour contained in a saturated space is compressed, a part of it returns to the liquid state; and that if, on the contrary, the pressure is diminished, a portion of the liquid remaining in excess is vaporised, and the space occupied by the vapour is saturated anew; but in both cases, the tension and the density remain constant. If the space where the vapour is contained be not saturated, or if it do not contain liquid in excess, the vapour, when the pressure increases or diminishes, acts entirely as a gas; that is, so long as it is not brought up to the point of saturation, its tension and its density increase with the pressure. Consequently, it is evident that vapours, in a space not saturated, act according to the law of Mariotte.

Tension of the Vapour of Water below the Freezing Point.-In order to measure the elastic force of the vapour of water below 0° Centigrade, Gay-Lussac employed two barometric tubes filled with mercury and immersed in the same cistern. One of these, completely freed from air and humidity, was used to measure the pressure of the atmosphere: into the other a small quantity of water was introduced, and its barometric chamber was surrounded with a small jacket, in which was placed a frigorific mixture. By comparing the heights of the two barometers when the temperature of the frigorific mixture stood at different points of the scale, Gay-Lussac found that in the barometer which contained the water, the depression of the mercury, and consequently the tension of the vapour, were as follows:

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Hence it is inferred that, at very low temperatures, there is still vapour of water in the air.

Tension of the Vapour of Water from the Freezing to the Boiling Point.-We shall first give the process adopted by Mr. Dalton, of Manchester, who died in 1844, in order to determine the elastic force of the vapour of water from 0° to 100° Centigrade. He employed two barometric tubes A and B, fig. 188, which were immersed in an iron vessel full of mercury and placed over a furnace.

The barometer B was freed from air and humidity, and in the barometer A was put a small quantity of water. These two barometers were kept in a vessel of glass full of water, in the middle of which was immersed a thermometer T, which indicated the temperature of the liquid. By gradually heating the iron vessel, and consequently the water in the glass vessel, that which was in the tube was vaporised; and in proportion as the tension of the vapour increased, the mercury was lowered. Then, by marking degree after degree on the scale E, the depression which took place in the tube A, below the level B in the other tube, Mr. Dalton determined the elastic force of the vapour of water at every point of the thermometer between the freezing and the boiling points, and was the first to construct a table of the same, as follows:

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