I cannot but esteem myself very happy that the remarks which I recently presented in the Moniteur Scientifique on systems of chemical notations have given rise to the interesting article which precedes. I would like, however, to add a few remarks.
I found this fault with chemical equivalents, that they are not susceptible of a precise and general definition. M. Berthelot, while opposing this doctrine, seems to me to confirm it, as he was obliged to give a double definition. Sometimes these equivalents represent the ratios according to which bodies substitute themselves to one another; this is certainly a precise conception, which I have called true chemical equivalence, but it can only be applied to restricted cases. At other times, they are the ratios according to which bodies combine, or rather one of the ratios, which are multiples of one another, according to which combinations take place; in this case, the question of equivalence remains undetermined and more or less arbitrary. Besides, we may add that it is not always easy to see why one or the other of these definitions can be applied to the equivalent of a body. For instance, aluminium, compared to the metals which are most nearly related to it, those of the earths and alkaline earths, has a perfectly determinate value of substitution, and still this is not the value that has been chosen for its equivalent.
As M. Berthelot himself observes, we only differ in the opinion that each of us has formed on the part and relative importance which are to be ascribed to physical properties in the determination of equivalents. I may possibly have an exaggerated idea of the importance of these properties. But does not M. Berthelot, on his side, labor under a delusion when he thinks that chemical considerations alone are a sufficient guide to a chemist in this determination; is he not under the influence, which he has often discovered in his opponents, of a state of mind in which things seem very natural because we are accustomed to accept them? If the equivalent adopted for aluminium was not confirmed by the specific heat of this metal, by the vapor density of its chloride, and by numerous considerations of isomorphism, is he very sure that we would not hesitate between the formulas Al2O3 and AlO for alumina? What would make us suppose that hesitation would be permissible is the great number of bodies in which the determination of the equivalent has remained doubtful as long as physical properties have not served as guides, such, for instance, as in silicon, zirconium, glucinum, and the numerous group of the metals of cerite and gadolinite. The only thing I ask is to allow the same importance to physical properties in the case of bodies which occur with frequency.
M. Berthelot finds fault with me for being in apparent contradiction with myself, because in my researches on the specific heats of saline solutions, I doubled the molecular weights of some compounds, to better express the parallelism of the properties of some compound groups. It is true that, in comparing with one another the salts of the same acid, it has seemed to me more natural to refer their properties to equivalent quantities, or to quantities containing always the same proportion of acid. But if this has led me to group together two molecules of an alkaline chloride or nitrate, I was obliged, for the same reason, when taking the specific heats of sulphate of alumina or of alkaline phosphates to take, as unities of the weights of these salts, quantities which are really equivalent of other bodies, but which only represent fractions of the admitted equivalents of these bodies. Notwithstanding, M. Berthelot does not conclude that this is a proof that alumina should be written AlO and phosphoric acid PhO5/3, and that the equivalents of aluminium and phosphorus should be modified accordingly.
It may sometimes be interesting to compare certain properties of bodies by referring them to chemically equivalent weights, in cases where these correspond neither to molecular weights nor to equivalents usually adopted, but we cannot conclude from this that those weights ought to be adopted as symbols of notations.
Apart from all these things, I agree with M. Berthelot that it is not advisable to exaggerate the importance of these questions, the solution of which cannot affect the important laws and theories of chemistry, and about which we can only reach a conclusion when we have arrived at more complete knowledge of the molecular constitution of compound bodies. This constitution itself will be doubtless revealed to us by researches on molecular mechanics, such as those on Thermo-chemistry, through which this eminent scientist aids so powerfully the advancement of science.