« PreviousContinue »
of moving forces. When an elas- finity, and repulsion. The doctick substance impinges on another trine of affinity is the most imat rest, the latter acquires some portant, and the most interesting, principle, by which it is caused to in the whole range of chemistry. change its place. These actions, In fact, to know the affinities of though resulting from the opera- bodies, is to be acquainted with the tion of certain laws of matter, are science ; and we have a right to obviously not chemical. The con expect, therefore, in a work like struction of achromatick glasses this, that the laws, by which they depends on the different degrees are regulated, should be accurately of refrangibility of two species of detailed and clearly arranged. But that substance ; and the laws, by we look in vain for this generalisawhich they are regulated, make a tion. The account, we think, is part of dioptricks. The formation, confused and imperfect, and by no however, of these substances, the means calculated to give its readers nature of the ingredients, and the a correct idea of its importance. theory of their reciprocal action, Dr. Ewell, however, is an enemy result from the application of laws, to divisions, and we cannot expect which are strictly chemical. On them in a work, whose author dethe other hand, the province of clares them opposed to the uninatural history is to describe the form simplicity of nature.' On qualities of bodies, and, according repulsion he has said little. In to their degrees of similitude in fact, he has altogether denied the external characters, to reduce them existence of a repulsive principle to certain classes, and subdivide among the particles of bodies, and them into genera and species. But endeavours to support his opinion neither the naturalist, nor the ma- by this sage observation that thematician, nor the natural phi. the cause of the separation of such losopher, has any thing to do with bodies is a mechanical one, as in those actions, which, in certain cir- most cases will appear evident, and cumstances, result from the con- consequently it would be improper tiguity of heterogeneous particles, to conclude there is a repulsive and are followed by a change of principle.' p. 50. properties and relations. We thus Our author thèn proceeds to approximate to the true definition describe the nature and principles of chemistry. But it is not our of those elementary substances, to duty to supply, though it may be which has been applied the term to expose, the deficiencies of Dr. unconfinable. The first of these Ewell. From this view of the sub- is heat. We could not help reject, it will appear obvious, that marking the want of arrangement, our author has given an erroneous in describing the various relations statement of the nature of this sci- of caloric. The fears of Dr. ence, by confounding it with those, Ewell, that his subject would be which treat only on the general rendered perplexed, by many subproperties of matter. Having thus, divisions, have led him into an opas he supposes, established the de- posite errour ; and the account, finition of chemistry, he devotes a therefore, of this substance, seems few pages to the consideration of both confused and imperfect.the general laws of matter, in which Chemists have usually divided cahe briefly notices the attractions of loric into two kinds, viz. of comgravity and cohesion, chemical af- munication, and of transmission, or
radiation. Of the latter, however, But let us for a moment suppose, he has said nothing. In fact, he that bodies, placed in the focus of seems not to have been acquainted a burning glass, are not acted upwith the experiments of Mr. Les on by the calorific solar rays, it lic,* and of Count Rumford,t on may still be asked, in what way the different radiating power of the can light act by evolving their lasurfaces of different substances, al- tent caloric ? When metallic, though on this principle have been concrete oily substances are founded important improvements thus exposed, they are soon rein various arts, essential to the com- duced to a state of fusion, their fort of society. On the various capacities are increased, and they conducting powers, of different sub- consequently absorb caloric ; yet stances, and the application of this Dr. Ewell is made to say, that bod. knowledge to the purposes of life, ies, exposed to the rays of the sun, Dr. Ewell has written with accu have their capacities for heat inracy and judgment, although here, creased, and consequently are made as in many other places, we re to evolve the caloric, with which marked the copious extracts from they were combined. This is evithe chemical work of Mr. Accum. dently an absurdity ; for it is imBut, while considering the various possible to conceive, that the fluidmodes of generating or evolving ity, and consequent increase of cacaloric, we were not a little sur- pacity of a body, could be producprised to find, that the heat, pro- ed by the evolution of a quantity of duced by the appulsion of the solar caloric, which was just sufficient rays on terrestial bodies, should be for it in a solid state. Our author considered as the effect of chemic- endeavours, in the same way, to al action. We are told,g that the account for the fusion and comcaloric, generated by bringing the bustion of different substances, by rays of the sun to a focus, by a con- the electrick and galvanick fluids, vex lens, is in consequence of the not recollecting that their capaciunion of light with the particles of ties for heat are probably diminishbodies, by which their capacity is ed by the sudden and violent com. destroyed, and their latent heat pression of their particles. consequently evolved. In proof In the subsequent pages we find of the correctness of his opinion, him attacking the theory of latent he affirms, that it requires some heat, as described by Dr. Black. time for the rays of caloric to pass He however quotes no authorities, through the substance of the glass. nor has once mentioned the names It is well known, that caloric is at of Crawford and of Irvine, who are first retarded in its passage through so justly celebrated for their writdiaphanous bodies, in consequence ings on the capacities of bodies. of combining with their particles. But we shall have sufficient reason At least, this is the fact with re to believe, that respect for the ogard to the radiation of the parti. pinions of others is not to be rankcles of this substance from culinary ed among the infirmities of Dr. fire, though the retardation of solar Ewell. It is quite unnecessary, heat is by no means so obvious. we presume, to enter on the dis
cussion of the different theories,
which have been advanced by sev* Leslie on heat. + Nicholson's Journal, 8vo. series.
eral philosophers on the nature of * Accum's Chemistry, vol. 1, p. 88.
capacity ; we shall, therefore, conPage 67.
fine ourselves to the examination
of those facts or experiments, on form a solid, and yet throw out which our author has grounded his no heat.' Now in the statement of opinion. On a review of these, this action, he has been guilty eiwe have sufficient reason to be ther of unpardonable negligence lieve, that he has not only been un or of great want of candour, that fortunate in his selection, but that he might bring it as a formidable they are calculated to establish argument against the truth of Dr. more strongly the doctrine which Black's theory. Had he taken the he is attempting to overthrow. trouble of consulting his oracle, Mr.
Nitric acid,' says he, in a Accum,* he would have found, that strong heat, is converted into two the union of ammoniacal gas with airs of great bulk, which contain ordinary muriatic acid gas is atno more heat than the acid did.' tended by a considerable' evoluThis is mere assertion. Dr. Ewell tion of calorick ; but when this alhas not pointed out the experi- kali is mixed with the oxy-muriatic ments, by which he is authorised acid, the combination is attended to state this fact with so much con- with a (rapid detonation, and acfidence ; nor indeed can he be in companied with a white fame.'t possession of any facts, on which The observation, that water gives he may ground his opinion. On out different quantities of caloric, the contrary, the decomposition of when rendered solid by freezing, the nitric acid is effected by the and by combining with quicklime, agency of caloric, and, at the mo.
is no proof in favour of our author's ment of transition of this substance hypothesis. Having given our from a liquid to a gaziform state, chemical readers an account of Dr. a considerable absorption of calor. Ewell's objections to the theory of ic might take place, without be- Dr. Black, we shall now present ing perceptible to the
them with his own hypothesis, and These two airs are azote and oxy- leave thein to form their own opingen, and it has been proved by ac ions of its truth. He states his tual experiment that both, in passa opinion to be, that the solidity ing from the aëriform to the liquid and Auidity of bodies depend on the or solid state, give out considerable same cause which varies their quantities of caloric. In proof of capacities for heat, and this is, the which our author has only to con exercise the particular affinities sult the papers of Mr. Biot* and of of the body, in the circumstances Mr. Northmore. The former of existing in the different degrees of these, by mechanical pressure, ef- heat.' p. 75. fected the union of oxygen and The discourse on heat is termi. hydrogen gases, which was atten- nated by an account of some exa ded by a strong luminous explo- periments, instituted by our author sion. It has been observed, that to ascertain the ponderability of the mere compression of atmos caloric. From the manner, in pherick air in the reservoir of an which these were conducted, thor air-gun has been followed by the they have been addressed to that production of light and heat. Dr. celebrated chemist, Dr. Mitchell, Ewell observes, that when we now of the United States senate,'t bring two airs together, the am and published in the Medical Remoniacal and the muriatic, they pository, we do not hesitate to say,
* Nicholson's Journal, vol. 12. p. 212. # Ibid. vol. 12 & 13. p. 361 et 238.
* Accum's Chem. vol. 1. p. 332.
I P. 75–6.
that they are of little weight in de- experiments, is not far from that termining the question, respecting of a cubick inch of atmospherick the nature of heat, which has so air ; yet, is it possible to believe, long divided chemical philoso- that this substance, which from its phers. We ought to observe, that excessive tenuity has been termed these experiments were made to unconfinable, can equal in specifick prove the corporeal nature of ca- gravity another, which is confinaloric ; for if heat can be weighed, ble, almost tangible, and as easily the question respecting its mate- managed as many liquid bodies? riality is at once decided. They If the proof of the materiality of consisted in dissolving salt in was caloric depended solely on these ter, and weighing the solution after experiments, the advocates of the the absorption of caloric had ceas. contrary theory, we think, would ed; 2dly, in adding sulphuric acid have little trouble in establishing to water and ascertaining the weight their positions. of the compound when it had fallen In common with most systemat. to the temperature of the ingredi- ick writers on chemistry, Dr. Ewell ents before they were mixed. The commencés his discourse on light conclusions, drawn from these ex with a detail of its physical properperiments, are very remarkable. sies. In speaking however of the He affirms, that they gained or lost refraction, which the solar rays sufhalf a grain' in weight for every fer in their passage from a rare to ounce of the mixture, and his in a dense medium, he has committed ference, presuming on their accu an important errour, which has racy, is very natural, that heat is probably escaped detection merely matter.' Now experiments to de- from the inattention of the author. termine this question were made In proof of the refractive power of so long ago as the age of De Luc the atmosphere he affirms, that of Geneva, and they have been re- , • the sun sets and rises earlier, than peated and varied by Lavoisier, Dr. appears to spectators.' Now, alFordyce, and Count Rumford, with most the converse of the proposithe most scrupulous exactness,with- tion is true. In consequence of out any general acquiescence in the refraction, which the rays of the belief of either doctrine. Those light suffer in their passage from a of the latter were characterised by rarer medium through our atmos. delicacy of instruments and accu- phere, the sun is apparently above, racy of calculation. Yet notwith- before he has actually reached, the standing this combination of cir. plane of the horizon. Dr. Ewell cumstances, which promised some surely cannot be ignorant of this positive conclusion, he was unable fact, nor that the degrees of refracto detect either the accession, or tion at different altitudes have been loss of weight, in the heating or accurately calculated by several cooling of bodies. Consequently, eminent astronomers, particularly the inference which he drew was, by Sir I. Newton, Mr. Simpson,
that neither the addition nor ab- and Dr. Bradley, whose tables may straction of caloric, makes any be seen in any elementary work on sensible alteration in the weights astronomy. Consequently, the sun of bodies.'* The weight which rises later and sets earlier, than apDr. Ewell gives to caloric, in his pears to the inhabitants of the
earth.* We are now to view our • Thompson's Chemistry, 2d ed. vol. 1. p. 308. Nicholson's Journal, 4to. se. * Enfield's Institutes, p. 256—7. Ferries, vol. 3. p. 381.
guson's Astronomy, 4th ed. p. 92.
author as the opponent of the illus- and the laws, by which it is governtrious Newton. We are always ed in its passage through diaphatempted to suspect, that when a nous, and its reflection from opake young man commences an attack' bodies. It is true, that black bodies on universally received doctrines, are seen by reflection, but this light which, in fact, are considered by is reflected from coloured bodies the world as axioms in philosophy, in their neighbourhood. When a he is actuated more by vanity,than black object is placed so as to ina the genuine spirit of philosophical tercept a portion of light, those inquiry. Dr. Ewell has underta- rays, which pass along ils edges, ken to overthrow the theory of the define its figure and mark its bounprismatick colours, and to erect on daries ; and it is thus more by the its ruins his own more ingenious interception of the rays of light hypothesis. . But we recommend from other bodies, that black subto hin the observation of the an stances are seen, than by its reflec
tion from their own surfaces. 2. "Non omnia possumus omnes.' The reason why diaphanous bodies He may be a good chemist, but he do not appear black is, that they is no optician. It may not be amiss, are capable of transmitting the rays however, to notice the arguments, of light, which Dr. Ewell thinks which he adduces to disprove the should produce that colour. While correctness of these principles. He the sun is above the horizon, an imthinks, that the experiment of de mense body of light is continually composing a ray of light by the re- flowing from different objects. fractive power of the prism, and These secondary, or reflected rays,, the subsequent union of the prim- passing in all directions, must neitive colours so as to produce the cessarily, in some cases, strike uporiginal ray, by no means conclu on these diaphanous bodies and be sive. · He goes on to observe, that either reflecied or transmitted, in “if the doctrine of the composition consequence of which the latter of light were true, it could not ac will appear more, or less coloured. count for all the colours of bodies. It is a well known fact, that objects To suppose, that blackness is the are always seen more or less disconsequence of the absorption of all tinctly thro' such substances arising the light must be absurd ; since it from the less or greater refraction is only by the reflection of light of these accidental rays of light. 3. that we are 'enabled to see black It is not necessary to the truth of bodies ; and since those, which are the Newtonian doctrine of colours, transparent, and of course allow the that bodies must reflect all the light light to pass through them, are which fall on their surfaces. It is very far from being black. That only necessary, that that portion, whiteness does not proceed from which is reflected, should be undethe reflection of all the light is compounded, or be still composed shown by the circumstance, that of the seven prismatick colours. the whiteness of bodies is not in Consequently a body may appear proportion to their reflection of white, which still transmits a porlight. Hence mercury, polished tion of light, impelled on its sur. iron, and other metals, reflect more face. The truth of this observalight, than the whitest paper.' tion has been demonstrated. AcThe futility of these objections will cording to the photometric exappear obvious to all, who are ac- periments of Mr. Leslie, of 100 quainted with the doctrine of light, parts of incident light vellum pa