of moving forces. When an elastick substance impinges on another at rest, the latter acquires some principle, by which it is caused to change its place. These actions, though resulting from the operation of certain laws of matter, are obviously not chemical. The construction of achromatick glasses depends on the different degrees of refrangibility of two species of that substance; and the laws, by which they are regulated, make a part of dioptricks. The formation, however, of these substances, the nature of the ingredients, and the theory of their reciprocal action, result from the application of laws, which are strictly chemical. On the other hand, the province of natural history is to describe the qualities of bodies, and, according to their degrees of similitude in external characters, to reduce them to certain classes, and subdivide them into genera and species. But neither the naturalist, nor the mathematician, nor the natural philosopher, has any thing to do with those actions, which, in certain circumstances, result from the contiguity of heterogeneous particles, and are followed by a change of properties and relations. We thus approximate to the true definition of chemistry. But it is not our duty to supply, though it may be to expose, the deficiencies of Dr. Ewell. From this view of the subject, it will appear obvious, that our author has given an erroneous statement of the nature of this science, by confounding it with those, which treat only on the general properties of matter. Having thus, as he supposes, established the definition of chemistry, he devotes a few pages to the consideration of the general laws of matter, in which he briefly notices the attractions of gravity and cohesion, chemical af

finity, and repulsion. The doctrine of affinity is the most important, and the most interesting, in the whole range of chemistry. In fact, to know the affinities of bodies, is to be acquainted with the science; and we have a right to expect, therefore, in a work like this, that the laws, by which they are regulated, should be accurately detailed and clearly arranged. But we look in vain for this generalisation. The account, we think, is confused and imperfect, and by no means calculated to give its readers a correct idea of its importance. Dr. Ewell, however, is an enemy to divisions, and we cannot expect them in a work, whose author declares them opposed to the uniform simplicity of nature.' On repulsion he has said little. In fact, he has altogether denied the existence of a repulsive principle among the particles of bodies, and endeavours to support his opinion by this sage observation that the cause of the separation of such bodies is a mechanical one, as in most cases will appear evident, and consequently it would be improper to conclude there is a repulsive principle.' p. 50.

Our author then proceeds to describe the nature and principles of those elementary substances, to which has been applied the term unconfinable. The first of these is heat. We could not help remarking the want of arrangement, in describing the various relations of caloric. The fears of Dr. Ewell, that his subject would be rendered perplexed, by many subdivisions, have led him into an opposite errour; and the account, therefore, of this substance, seems both confused and imperfect.Chemists have usually divided caloric into two kinds, viz. of communication, and of transmission, or

radiation. Of the latter, however, he has said nothing. In fact, he seems not to have been acquainted with the experiments of Mr. Leslie, and of Count Rumford,† on the different radiating power of the surfaces of different substances, although on this principle have been founded important improvements in various arts, essential to the comfort of society. On the various conducting powers, of different substances, and the application of this knowledge to the purposes of life, Dr. Ewell has written with accuracy and judgment, although here, as in many other places, we remarked the copious extracts from the chemical work of Mr. Accum.‡ But, while considering the various modes of generating or evolving caloric, we were not a little surprised to find, that the heat, produced by the appulsion of the solar rays on terrestial bodies, should be considered as the effect of chemical action. We are told,§ that the caloric, generated by bringing the rays of the sun to a focus, by a convex lens, is in consequence of the union of light with the particles of bodies, by which their capacity is destroyed, and their latent heat Consequently evolved. In proof of the correctness of his opinion, he affirms, that it requires some time for the rays of caloric to pass through the substance of the glass. It is well known, that caloric is at first retarded in its passage through diaphanous bodies, in consequence of combining with their particles. At least, this is the fact with regard to the radiation of the particles of this substance from culinary fire, though the retardation of solar heat is by no means so obvious.

Leslie on heat.

Nicholson's Journal, 8vo. series. Accum's Chemistry, vol. 1, p. 88. Page 67.

or

But let us for a moment suppose, that bodies, placed in the focus of a burning glass, are not acted upon by the calorific solar rays, it may still be asked, in what way can light act by evolving their la tent caloric ? When metallic, concrete oily substances are thus exposed, they are soon reduced to a state of fusion, their capacities are increased, and they consequently absorb caloric; yet Dr. Ewell is made to say, that bodies, exposed to the rays of the sun, have their capacities for heat increased, and consequently are made to evolve the caloric, with which they were combined. This is evidently an absurdity; for it is impossible to conceive, that the fluidity, and consequent increase of capacity of a body, could be produced by the evolution of a quantity of caloric, which was just sufficient for it in a solid state. Our author endeavours, in the same way, to account for the fusion and combustion of different substances, by the electrick and galvanick fluids, not recollecting that their capacities for heat are probably diminished by the sudden and violent compression of their particles.

In the subsequent pages we find him attacking the theory of latent heat, as described by Dr. Black. He however quotes no authorities, nor has once mentioned the names of Crawford and of Irvine, who are so justly celebrated for their writings on the capacities of bodies. But we shall have sufficient reason to believe, that respect for the opinions of others is not to be ranked among the infirmities of Dr. Ewell. It is quite unnecessary, we presume, to enter on the discussion of the different theories, which have been advanced by several philosophers on the nature of capacity; we shall, therefore, confine ourselves to the examination

of those facts or experiments, on which our author has grounded his opinion. On a review of these, we have sufficient reason to believe, that he has not only been unfortunate in his selection, but that they are calculated to establish more strongly the doctrine which he is attempting to overthrow. Nitric acid,' says he, in a strong heat, is converted into two airs of great bulk, which contain no more heat than the acid did.' This is mere assertion. Dr. Ewell has not pointed out the experiments, by which he is authorised to state this fact with so much confidence; nor indeed can he be in possession of any facts, on which he may ground his opinion. On the contrary, the decomposition of the nitric acid is effected by the agency of caloric, and, at the moment of transition of this substance from a liquid to a gaziform state, a considerable absorption of caloric might take place, without being perceptible to the senses. These two airs are azote and oxygen, and it has been proved by actual experiment that both, in passing from the aëriform to the liquid or solid state, give out considerable quantities of caloric. In proof of which our author has only to consult the papers of Mr. Biot* and of Mr. Northmore. The former of these, by mechanical pressure, effected the union of oxygen and hydrogen gases, which was attended by a strong luminous explosion. It has been observed, that the mere compression of atmospherick air in the reservoir of an air-gun has been followed by the production of light and heat. Dr. Ewell observes, that when we bring two airs together, the ammoniacal and the muriatic, they

Nicholson's Journal, vol. 12. p. 212. Ibid. vol. 12 & 13. p. 361 et 238.

form a solid, and yet throw out no heat.' Now in the statement of this action, he has been guilty either of unpardonable negligence or of great want of candour, that he might bring it as a formidable argument against the truth of Dr. Black's theory. Had he taken the trouble of consulting his oracle, Mr. Accum,* he would have found, that the union of ammoniacal gas with ordinary muriatic acid gas is attended by a considerable' evolution of calorick; but when this alkali is mixed with the oxy-muriatic acid, the combination is attended with a rapid detonation, and accompanied with a white flame.'t The observation, that water gives out different quantities of caloric, when rendered solid by freezing, and by combining with quicklime, is no proof in favour of our author's hypothesis. Having given our chemical readers an account of Dr. Ewell's objections to the theory of Dr. Black, we shall now present them with his own hypothesis, and leave them to form their own opinions of its truth. He states his opinion to be, that the solidity and fluidity of bodies depend on the same cause which varies their capacities for heat, and this is, the exercise of the particular affinities of the body, in the circumstances existing in the different degrees of heat.' p. 75.

that they are of little weight in determining the question, respecting the nature of heat, which has so long divided chemical philosophers. We ought to observe, that these experiments were made to prove the corporeal nature of caloric; for if heat can be weighed, the question respecting its materiality is at once decided. They consisted in dissolving salt in water, and weighing the solution after the absorption of caloric had ceas ed; 2dly, in adding sulphuric acid to water,and ascertaining the weight of the compound when it had fallen to the temperature of the ingredients before they were mixed. The conclusions, drawn from these experiments, are very remarkable. He affirms, that they gained or lost half a grain in weight for every ounce of the mixture, and his inference, presuming on their accuracy, is very natural, that heat is matter.' Now experiments to determine this question were made so long ago as the age of De Luc of Geneva, and they have been repeated and varied by Lavoisier, Dr. Fordyce, and Count Rumford, with the most scrupulous exactness, without any general acquiescence in the belief of either doctrine. Those of the latter were characterised by delicacy of instruments and accuracy of calculation. Yet notwithstanding this combination of cir cumstances, which promised some positive conclusion, he was unable to detect either the accession, or loss of weight, in the heating or cooling of bodies. Consequently, the inference which he drew was, that neither the addition nor abstraction of caloric, makes any sensible alteration in the weights of bodies.'* The weight which Dr. Ewell gives to caloric, in his

Thompson's Chemistry, 2d ed. vol. 1. p. 308. Nicholson's Journal, 4to. series, vol. 3. p. 381.

experiments, is not far from that of a cubick inch of atmospherick air; yet, is it possible to believe, that this substance, which from its excessive tenuity has been termed unconfinable, can equal in specifick gravity another, which is confinable, almost tangible, and as easily managed as many liquid bodies? If the proof of the materiality of caloric depended solely on these experiments, the advocates of the contrary theory, we think, would' have little trouble in establishing their positions.

In common with most systemat ick writers on chemistry, Dr. Ewell commences his discourse on light with a detail of its physical properties. In speaking however of the refraction, which the solar rays suffer in their passage from a rare to a dense medium, he has committed an important errour, which has probably escaped detection merely from the inattention of the author. In proof of the refractive power of the atmosphere he affirms, that

the sun sets and rises earlier, than appears to spectators.' Now, almost the converse of the proposition is true. In consequence of the refraction, which the rays of light suffer in their passage from a rarer medium through our atmosphere, the sun is apparently above, before he has actually reached, the plane of the horizon. Dr. Ewell surely cannot be ignorant of this fact, nor that the degrees of refraction at different altitudes have been accurately calculated by several eminent astronomers, particularly by Sir I. Newton, Mr. Simpson, and Dr. Bradley, whose tables may be seen in any elementary work on astronomy. Consequently, the sun rises later and sets earlier, than appears to the inhabitants of the

earth. We are now to view our

* Enfield's Institutes, p. 256-7. Ferguson's Astronomy, 4th ed. P. 92.

author as the opponent of the illustrious Newton. We are always tempted to suspect, that when a young man commences an attack on universally received doctrines, which, in fact, are considered by the world as axioms in philosophy, he is actuated more by vanity,than the genuine spirit of philosophical inquiry. Dr. Ewell has undertaken to overthrow the theory of the prismatick colours, and to erect on its ruins his own more ingenious hypothesis. But we recommend to him the observation of the ancient poet,

'Non omnia possumus omnes.' He may be a good chemist, but he is no optician. It may not be amiss, however, to notice the arguments, which he adduces to disprove the correctness of these principles. He thinks, that the experiment of decomposing a ray of light by the refractive power of the prism, and the subsequent union of the primitive colours so as to produce the original ray, by no means conclusive. He goes on to observe, that "if the doctrine of the composition of light were true, it could not account for all the colours of bodies. To suppose, that blackness is the consequence of the absorption of all the light must be absurd; since it is only by the reflection of light that we are enabled to see black bodies; and since those, which are transparent, and of course allow the light to pass through them, are very far from being black. That whiteness does not proceed from the reflection of all the light is shown by the circumstance, that the whiteness of bodies is not in proportion to their reflection of light. Hence mercury, polished iron, and other metals, reflect more light, than the whitest paper.' The futility of these objections will appear obvious to all, who are acquainted with the doctrine of light,

2.

and the laws, by which it is governed in its passage through diaphanous, and its reflection from opake bodies. It is true, that black bodies are seen by reflection, but this light is reflected from coloured bodies in their neighbourhood. When a black object is placed so as to intercept a portion of light, those rays, which pass along its edges, define its figure and mark its boundaries; and it is thus more by the interception of the rays of light from other bodies, that black substances are seen, than by its reflection from their own surfaces. The reason why diaphanous bodies do not appear black is, that they are capable of transmitting the rays of light, which Dr. Ewell thinks should produce that colour. While the sun is above the horizon, an immense body of light is continually flowing from different objects. These secondary, or reflected rays,, passing in all directions, must necessarily, in some cases, strike upon these diaphanous bodies and be either reflected or transmitted, in consequence of which the latter will appear more, or less coloured. It is a well known fact, that objects are always seen more or less distinctly thro' such substances,arising from the less or greater refraction of these accidental rays of light. 3. It is not necessary to the truth of the Newtonian doctrine of colours, that bodies must reflect all the light which fall on their surfaces. It is only necessary, that that portion, which is reflected, should be undecompounded, or be still composed of the seven prismatick colours. Consequently a body may appear white, which still transmits a portion of light, impelled on its surface. The truth of this observation has been demonstrated. According to the photometric experiments of Mr. Leslie, of 100 parts of incident light vellum pa