of the bottle, plus the weight of the gas, minus the weight of the air displaced, according to principles formerly explained. Now, the weight of the bottle is easily determined; and if it be gauged by finding the volume of water which it contains, its volume will thus be found, and consequently the weight of the air which it displaces. If, then, we subtract the weight of the bottle itself from the weight obtained by weighing it when full of gas, and add to the remainder the weight of the air displaced, we have the required weight of the gas. It now only remains to divide the weight of the gas by the weight of the same volume of air, care-being taken to make the necessary corrections for temperature and pressure in order to refer them to the same capacity, and the standard temperature and pressure. The following table shows the density of some gases which have been thus determined at the standard temperature and pressure above mentioned:

TABLE OF THE DENSITY OF GASES,

CHANGE OF STATE IN BODIES.

The Laws of Fusion.-Of the various phenomena which are presented by bodies under the influence of caloric, we have only hitherto considered those of expansion. Now, if we first turn our attention to solid bodies, it is evident that this expansion has a limit. For, in proportion as a solid body absorbs a greater quantity of caloric, the repulsive force of its particles is increased; and a period may arrive when the molecular attraction is insufficient to preserve the body in the solid state. A new phenomenon then takes place; viz. that of fusion (melting), or the passage of a body from the solid to the liquid state. Yet a great number of substances, as paper, wood, wool, and certain salts, do not melt under the action of an elevated temperature, but are decomposed. Of all the simple bodies, one only has not hitherto been fused by the action of the most intense sources of heat, and this is carbon.

The following are the two general laws of fusion to which bodies are subjected, as discovered by experiment. 1. Every body enters into a state of fusion at a certain temperature, which is invariable for each individual substance. 2. What ever may be the intensity of the source of heat at the moment when fusion commences, the temperature ceases to rise, and remains constant until the fusion be completed. The following table exhibits the temperatures at which fusion commences in different substances :

TABLE OF THE MELTING POINTS OF BODIES.
Substances.
Melting Points.
40° Cent. or 40° Fahr.
32
91.4

Mercury

Ice

33

Tailow

Phosphorus
Potassium

Margaric Acid

Stearin

White wax

Stearic Acid

Alloy (1 lead, 1 tin,

4 bismuth) tin,}

Sodium

145 4

201.2,,

Latent Caloric.-When the temperature remains constant during the period of fusion, while a body passes from the solid to the liquid state, whatever may be the intensity of the fire, it is evident that in order to change its state, the body absorbs a considerable quantity of heat, of which the sole effect is to keep it in the liquid state. This quantity of heat, which does not act on the thermometer, and which is combined in some way with the particles of the body, is denominated latent caloric, or the caloric of fusion. The following experiment will give a clear idea of what is to be understood by latent caloric: if we mix a pound of water at 0° Centigrade with a pound of water at 79 Centigrade, we shall have immediately two pounds of water at 39.5 Centigrade, that is, at the mean temperature of the two quantities mixed; this result was to be expected, because they are of the same nature, and of an equal quantity. But if we mix a pound of water at 79° Centigrade with an equal weight of pounded ice at 0° Centigrade, the ice will instantly melt, and we shall have two pounds of water at 0° Centigrade. Thus we see that, without changing its temperature, and solely to effect its fusion, one pound of ice absorbs the quantity of heat necessary to raise one pound of water from the freezing point to 79° Centigrade, or 1649.4 Fahrenheit. This quantity of heat, therefore, represents the caloric of fusion, or the latent caloric of ice. Each body has its own particular quantity of latent caloric, which may be determined by calculation.

Solution.-A body is said to be dissolved, or put into a state of solution, when it is liquified by the effect of the mutual attraction of its particles and those of a liquid. Thus, gum arabic, sugar, and the greater number of salts, are soluble in water. During solution, as well as during fusion, a greater or less quantity of heat is absorbed. This is the reason why the solution of a salt, in general, occasions a lowering of temperature. Yet it happens that in certain solutions the temperature does not vary, and in others that it even rises. This will be understood by observing how these two simultaneous and contrary effects are produced. The first is the passage from the solid to the liquid state, an effect which produces a lowering of temperature; the second is the combination of the dissolved body and the liquid. Now, every chemical combination takes place with the development of heat; consequently, according as one of these effects predominates over the other, or as one is equal to the other, so is cold or heat the result, or the temperature remains constant.

Solidification.-Solidification, or congelation, is the passage from the liquid state to the solid. This phenomenon takes place according to the following laws, which are the converse of those of fusion, and are proved by experiment: 1. Solidification is produced in every body at a fixed temperature, which is exactly that of fusion. 2. From the moment when solidification commences until it be completed, the temperature of the liquid remains constant. This second law is the consequence of the fact that the latent caloric absorbed during fusion is set free at the moment of solidification. Many liquids, as alcohol and ether, are not solidifiable by the greatest lowering of temperature to which they have been exposed. In general, bodies which pass slowly from the liquid to the solid state assume determinate geometrical forms called crystals; such as those of the tetrahedron, the cube, the prism, and the rhombohedron. If the body which solidifies be in a state of fusion, its crystallisation is said to take place by the dry method; but if the body be held in solution in a liquid, its crystallisation is said to take place by the humid method. It is by allowing the liquids which hold salts in solution to evaporate slowly, that salts are made to crystallise. Snow, ice newly formed, and salts, exhibit fine examples of crystallisation. We shall take the first opportunity to bring under the notice of our students the subject of crvstallisation.

The Formation of Ice.-Distilled water becomes solid at the temperature of 0 Centigrade, and is then called ice; but the

congelation takes place but slowly, because that the frozen part gives out its latent caloric to the rest of the liquid mass. Ice presents this remarkable phenomenon, that it possesses less density than water. We have already shown that, by cooling or lowering the temperature, water only contracts in volume as far as 40 Centigrade, but that beyond this point it expands. Now, this increase of volume remains and increases still more at the moment of congelation; and we find that the volume of ice is 1.075 times that of water at 4° Centigrade. Hence, the density of ice is only about 0.930, that of water being 1; consequently, ice always floats on the surface of water. The increase of volume which ice assumes in its formation is accompanied with a considerable expansive force, which frequently bursts the vessels which contain it. The rending of stones after a frost is due to the effect of the water which has penetrated their pores and become frozen. It is the same increase of volume which renders the action of frost so injurious to plants, because their sap, when frozen, breaks their tissues. M. Williams, in England, in order to demonstrate the expansive force of ice, placed in an atmosphere several degrees below zero, a bomb-shell filled with water, after he had firmly closed the orifice with a wooden stopper. At the instant of congelation, this stopper was forcibly thrown to a great distance, and an icy border was formed round the edges of the orifice.

Retardation of the Freezing of Water.-The temperature of the congelation of water is retarded by salts or other substances which it holds in solution. Sea-water, for instance, does not solidify till it be lowered to the temperature of -29.5 Centigrade, or 27.5 Fahrenheit. The point of the solidification of pure water may be retarded several degrees, if it be deprived of the air which it generally contains, and if it be kept entirely free from all agitation. Thus, in a vessel surrounded with a frigorific mixture, and placed under an exhausted receiver, the water may be made to fall to -12° Centigrade, or 10°*4 Fahrenheit, and even lower than this before congelation. But if then a slight motion be given to the mass, a part of the liquid will be instantly frozen; and this remarkable phenomenon will be observed, that the remaining part of the liquid will suddenly rise to 0 Centigrade, or 32° Fahrenheit. This rise in the temperature is owing to the latent caloric, which is freed by the formation of the ice.

LESSONS IN GREEK.-No. XXXI.

By JOHN R. BEARD, D.D.
CONTRACTED VERBS PURE.

CONTRACTED Pure Verbs are those which have for their characteristic either a, e, or o, and blend those vowels with the immediately following mood-vowel. The mixing of the vowels takes place in only the Present and Imperfect of the active and middle (or passive), since only in those two is the characteristic vowel followed by the modal vowel. The v EQEλKVOTIKOV in the third person singular, Imperfect, active, is not employed with the contraction.

The blending of two vowels produces various vowels or diphthongs, as appears in the following table, where +, the sign of plus in Mathematics, denotes that the two vowels between which it is placed, melt together to produce another or a diphthong, and =, the sign of equality, is prefixed to the result, showing that the latter is equivalent to the former.

Observe that Aovw, I wash, forms from the simpler verb λοw, the Middle Present λοῦται, λούμενος, λοῦ, λοῦσθαι ; the the other parts are Imperfect eλovμny, ελov, eλouto, etc.: regularly formed from Xovo.

I must now lay before you an example of the way in which Contracted Verbs are conjugated. In committing the ensuing forms to memory, you should repeat first the uncontracted form, and then the contracted form. The uncontracted form, to take an instance from the table, is raw, the contracted Tu: the uncontracted form is appended to the common stem in parentheses thus, ru(a-w); the contracted form stands immediately after the second parenthesis. This, then, is the way in which I advise you to repeat every part, in order to commit the whole to memory; namely,

Τιμαω, τιμῶ; τιμαεις, τιμᾷς; τιμαει, τιμᾷ τιμαετον, τιμᾶτον ; τιμάετον, τιμᾶτον ; τιμαομεν, τιμῶμεν; τιμαετε, τιμᾶτες τιμαουσι, τιμῶσι.

Τιμαομαι, τιμῶμαι; τιμαεσθω, τιμασθω; μισθοεσθων, μισθουσθων; φιλεόμενος, φιλούμενος; εφιλεόμεθον, ἔφιλουμεθον ; ετιμαεσθε, ετιμᾶσθε; εφιλέοντο, εφιλοῦντο, etc..