pound body, it completely difproves his conclufion. 2. No fe condary compound is gafeous. What is etherized nitrous gas? 3. None of them are combustible.' Spirit varnish is not com buftible! The detonating falts are not fecondary compounds!! 4. The fecondary compounds have been investigated with more precifion than any other clafs of bodies; from them almost all our notions of affinity have been derived; it is to them we have always recourse to illustrate thefe notions,' &c. &c. But of the primary compounds we were alfo told (vol. II. p. 263.), that they were the 'clafs of bodies which have been the longest known, which have been most accurately ftudied, and which conftitute, without doubt, the most important instruments of chemistry;' and, in the preface, it was mentioned as one of the characteristic merits of our author's ar rangement, that it begins with thofe parts which have been moft fuccefsfully investigated. But this kind of inconsistency is of very little confequence, if it at all promote our author's view of exciting the attention of his readers, by exaggerating the importance of every fubject which fucceffively engages them.

Having collected the immenfe mafs of facts contained in the two first books, our author now proceeds to treat, in the third, of thofe general laws by which the whole are regulated. Our imperfect knowledge of thefe, is afcribed by him partly to the unaccountable negligence of the greater number of chemifts, who have been more anxious to afcertain particular facts, than to investigate general principles, and who have often 'feemed to look upon general principles as altogether foreign to their science.' There may be fome truth in this obfervation; but, fuch an opinion, coming from fo high an authority, may be attended with very bad confequences, in misleading young men to waste their time and labour on idle fpeculations, and to defpife the lefs brilliant, but more fubftantial reputation, of increafing our ftore of facts. For our part, we are perfuaded that even Dr Thomson himself, notwithstanding the extent of his knowledge, and the univerfality of his talents, has done infinitely more fervice to chemistry by his industry as a compiler, and his affiduity in obferving the refults of mixtures, undirected by general views, than by all his attempts at generalization.

The first chapter of this book is faid to treat of Affinity in general. Many chemifts diflike the term affinity altogether; but with Dr Robifon we think it is of ufe, as it diftinguishes very compendiously the phenomena of combination (which are the chief objects of chemistry) from the phenomena of cohesion, adhefion, capillary attraction,' &c. In this limited fenfe alone, as fynonymous with chemical attraction, and in contradiftinction to cohesion and the other fpecies of attraction, has it been hitherto

employed,

employed. By Dr Thomfon, however, it is arbitrarily, and, we think, injudicioufly extended to include, as a generic term, every fpecies of contiguous attraction, and to comprehend those very forces from which it was invented-to difcriminate that attraction which is properly chemical. The neceflary confequence of this innovation is not only embarrassment to Dr Thomion's readers, but real ambiguity and confufion in his writings, where it is fometimes employed in the limited, and fometimes in the extended fig nification. Adhesion and cohesion are claffed together, as homogeneous affinities,' while chemical attraction is diftinguished by the phrase heterogeneous affinity.' But these innovations are at variance with fact; for heterogeneous bodies adhere, and, if we mistake not, cohere alfo, as in fome compound ftones. Since, therefore, heterogeneous bodies attract each other, independently. of combination, heterogeneous affinity is an inaccurate expreffion for chemical attraction.

Contiguous attraction is faid by our author to resemble fenfible attraction, in increasing with the mafs of the attracting bodies, and diminishing as the distance increafes. Of this, however, he is able to adduce no proof; and the refemblance must be therefore confidered as merely hypothetical. Indeed, he confeffes himfelf unable to determine, whether contiguous, like fenfible attraction, decreases in the ratio or in a greater ratio; but if it be at all proportionate to distance, it must follow a much greater ratio; for, at a distance greater than contiguity, it becomes altogether infenfible, or bears no proportion to the force of gravitation; whereas, whenever it becomes fenfible, it is more intenfe than gravitation. But the most important character of contiguous attraction is, that it varies in intenfity in different particles; on which occafion, our author indulges himself in fpeculating, whether it be one force, or many forces; whether it be owing to the figure of the particles, or whether it be the fame with gravitation; and after exerting all his ingenuity, he leaves himself and his readers just as wife as when he began. Cohefion is treated at confiderable length, according to the hypothefis of Bofcovich; and we are told, that it is deferving of notice, that the cohefive force of fimple bodies is greater than that of compounds, except in the cafe of the metals and elaftic fluids-that is, except in 26 cafes out of 29! To which lift of exceptions he fhould have added fulphur and phosphorus, which are not fo hard as most of the fulphats and phosphats: fo that diamond turns out to be the only fimple fubftance which is harder than all its compounds. Haüy's theory of cryftallization is very neatly stated; but the influence of the free accefs of air in promoting cryftallization, cannot be explained on the fuppofi

tion that it carries off caloric; for, upon that fuppofition, crystallization fhould take place at the fame temperature, whether the air be excluded or admitted.

We now come to the most important chapter in the whole work, that on Heterogeneous Affinity. From the arrangement adopted by our author, the expectations of his readers may reafonably be raised to the higheft pitch; but we are much afraid that their difappointment will be equally great, not, however, from any inability or negligence on his part, but from the view he has taken of the fubject. Instead of being fatisfied with ascertaining the general laws of combination by fair induction, he has treated chemistry as a mere branch of mechanical philofophy; he has confidered chemical attraction as the fame force with adhesion, and as fubject to the fame laws; and is fo completely abforbed in the attraction and repulfion of particles, that he lofes all fight of what is peculiar to chemistry, and only notices its laws accidentally in the courfe of his more general and abftrufe fpeculations. Thefe, we confefs, are not uninterefting, and may be acceptable to those who delight in what may be called philofophical intoxication, but appear to us extremely misplaced in an elementary work, which Thould be adapted to the capacity of all its readers; and, if it ever enter into such speculations, fhould treat them merely as of fe condary importance, and matters of curiofity. Our opinion, indeed, may be the effect of prejudice; for we may be mifled by the high authority of our inftructors in chemistry and mechanical philofophy, the one of whom thus fpeaks of the manner in which the other confidered this very fubject: The worthy author of thefe lectures was always more anxious to communicate what may be called a clear and confident knowledge of the doctrines of pure chemistry, than to lead his pupils into abftrufe or refined Ipeculations on the unfeen and unknown immediate caufes of chemical combination. He confidered every fuch question as rather cut of the pale of chemical fcience; and fo it certainly is. Whenever we fpeculate about the attractions and repulfions of particles, as the immediate agents in effecting the chemical changes, we are no longer chemifts, but mechanicians. We are confidering questions about local motion, and the mathematical determinations of the effects of moving forces. Not only is the occupation not chemical, but the questions themselves give little addition of chemical knowledge.

Dr Thomfon, however, thinks otherwife; and it is our duty to follow him in his fpeculations; the first of which is, that he confiders it very probable, that there exifts a reciprocal affinity between every fpecies of the particles of bodies. But

Black's Lectures, by Profeffor Robifon, vol. I. p. 512.

his

his proofs of its exiftence in thofe numberlefs cafes where it is commonly denied, are very unfatisfactory. For, the folution of foap in water, and lime in nitric acid, certainly do not prove that oil has any affinity for water, or lime for azote. In this laft cafe, as well as in many others, a fubftance is found to have a strong affinity for a compound, which, in every circumstance, refufes to unite with either of its conftituents; while, on the contrary, there are also numerous examples of fubftances refusing to unite with a compound, which have a ftrong affinity for its constituents. But this change of property, which is the strongest character of chemical action, feems to be totally overlooked by our author in all his reafonings about affinity. Bodies are in general believed to differ in the intensity of their affinity for each other; and M. Berthollet has lately fhewn, that this is much modified by their comparative maffes. But it by no means follows from the nature of affinity, that if a particle A attract B with a force = x, that two particles A ought to attract B with a force at leaftyx; for B may unite with one particle A, and form a compound C, which has no affinity for a fecond particle A. The fame argument is equally conclufive against the opinion fupported by our author-that dif ference of intenfity of affinity is infufficient to account for decompofition, unless fome other force, fuch as elafticity or cohesion, intervene to determine the exclufion of fome particular bodies. Indeed, if this opinion be true, when compound bodies unite, the combination does not take place between them as compounds, but amongst the elementary particles of which they are compofed and no fuch thing as a fecondary compound can exift. Saturation is fufficiently well defined-the balancing of affinity with its anta gonist forces, cohesion and elafticity. It is owing to this that the freezing point of water is lowered when it holds fome bodies in folution. But our author carries his reafoning rather a little too far, when he concludes that a table of the freezing points of different faline folutions would be a pretty accurate indication of the affinity of the different falts for water. On this principle, how will he account for the fact, that fulphuric acid, combined with a certain proportion of water, actually raises its freezing point, but with a larger quantity lowers it confiderably? And as the fame reafoning ought to apply to vaporization, how comes the boiling point of fome faline folutions to be lower than that of water? Neutralization takes place, when bodies unite in fuch proportions that they mutually deftroy or difguife the properties of each other. In this ftate, our author fuppofes their combination to be as perfect as poffible, and that their affinities are equal, that is, that the affinity of A for B is equal to that of B for A, He

next proceeds to demonstrate, that, in all combinations, there is a maximum and minimum in the proportions of the constituents, beyond which they can never país; but he cannot determine whe ther they are capable of combining in any indefinite proportion between thefe limits, or only in certain determinate proportions. In the latter cafe, therefore, with unufual caution he confults experience; and he certainly would have acted more wifely to have done the fame in the former cafe; for his reafoning is founded on principles purely hypothetical, and leads him to conclufions directly contrary to fact-for example, that elastic bodies can only combine with each other in one proportion. Now, azotic gas combines with oxygen gas in four proportions; and the proportions of the carbonates of animonia are the most unsteady of all the crystallizable falts. We are alfo told, that all compounds, of which the ingredients combine only in certain determinate proportions, have an elaftic fluid for one of the ingredients; yet we have the tartrat and fuper-tartrat of potafs, the fulphurets and fuper-fulphurets of the metals, &c.

We now come to the confideration of the various methods which have been propofed to exprefs the ftrengh of every affinity in numbers. The first that meets with our author's approbation, is that of Morveau, founded on the fuppofition that the affinity of bodies for each other is directly as the force neceflary to overcome the adhesion of their furfaces. But, befides the impracticability of carrying it into effect, which even the ingenious fuggeftions of our author will not remove, it is merely hypothetical, and cannot be admitted unless it be found to coincide with fact. But a disk of glafs adhered to water with a force of 258 grains, and to a folution of potafs, though denser, only with a force of 210; yet water has no chemical action on glafs, and a folution of potafs has. From a series of hypothetical principles, Berthollet concluded, that the affinities of bodies were inverfely as the mafs of each body capable of neutralizing the other; and, to bring this conclufion to the test of experience, our author has calculated the affinities of the acids and bafes for each other from Kirwan's laft table of the falts; from which he concludes, that it is exceedingly probable that the real order of affinities does not deviate far from that given in his tables, derived from these calculations. Now, the best way of afcertaining the probability of fuch an hypothesis, is to compare it with the facts. Accordingly, this has been very properly done by Dr Thomfon; and he finds that the affinity of the bafes for the acids follow precifely the inverfe order of that given by Bergman., This objection, however, is of little importance; for Bergman trufted to the clumfy mode of experiment, by afcertaining what falts decompofed each other;

and