572 REPORT— 1881. 



predicted implicitly by the atomic theory of the constitution of the elements; and, 

 wherever the physical properties of the molecules are such as aft'ord any basis for 

 the determination of their relative weights, such results cagree with those derived 

 from purely chemical considerations guided by the atomic tbeory. 



Our knowledge of molecules is us yet in its infancy. Even among the com- 

 monest elements and compounds we know the molecular weights of very few, but 

 what we do know of them proves that the idea of compound atoms invented by 

 chemists to explain the elementary facts of chemical action is, as far as it goes, a 

 true representation of what exists in nature. 



Many of the molecules thus proved to exist were the same as those suggested 

 under the dualistic system ; but many were proved, by the more accurate and 

 extensive knowledge of their reactions and properties, to have different weights 

 from those which had been at first attributed to them, yet always consistent with 

 the fundamental requirements of the atomic theory. Thus H2O, CO, CO,,, CH^,, 

 SOj, SO3, CaO, FeO, FcoOj are the formulae still used to denote the molecules of 

 the respective compounds, though the last three ought probably to be represented 

 by some multiple. On the other hand, the molecule of defiant gas is now repre- 

 sented by the formula C^H^, instead of CH.j. The chloracetate is C^ClaHOj, instead 

 of CjClg, C5O3, HjO. The molecule of benzoil chloride is C^H^OCl, instead of one 

 corresponding to (0,^115)203, C^H^CL, and chlorosulphuric acid is Cl^SO^, instead 

 of2S03, SClg. 



In proportion as chemists came to know more of the constitution of molecules^ 

 and to study chemical reactions from the point of view of the changes which they 

 bring about in the constitution of molecules, did the idea of substitution come to 

 be more and more used in the place of that of mere additive combination. A vast 

 number of processes of chemical combination, which had been considered as con- 

 sisting of direct combination, were found to be processes of double decomposition. 



One of the most important facts which was brought to light by the careful 

 examination of the composition of salts and organic bodies, aided by the molecular 

 method of representing their constitution, was that hydrogen is chemically one of 

 the metals, and that the compounds formed by the combination of water with 

 acids are analogous to other salts of those acids ; while compounds of hydrogen 

 with elements or radicals like chlorine are salts, analogous in their constitution to 

 other chlorides, &c. 



The molecular or unitary mode of viewing the constitution of each substance 

 affords more true as well as more simple records of the facts observed in chemical 

 reactions than could be obtained in the dualistic systems. A salt such as hydric 

 sulphate used to be considered as containing sulphuric acid and water, and repre- 

 sented by a formula such as SO3, H^O, implying the presence in it of both the 

 substances from which it was known to be formed. 



When two elements combined, their product was considered and described as- 

 containing the elementary atoms which had served to form it, and it was consistent 

 with this habit to represent a product which had been formed by the combination 

 of two compound molecules as containing those molecules. 



But the main business of chemical investigation is to observe accurately the 

 changes of composition which take place in the reactions of known substances, 

 with a view of discovering the atomic interchanges to which they are due. 



The compound formed by the combination of sulphuric acid and water differs 

 in many physical and chemical properties from both of those bodies. Its name and 

 its atomic formula serve to denote the aggregate of properties which are known 

 to belong to it, whereas the dualistic formula, SO3, H^O, served to recall the pro- 

 perties of the acid and base from which it was formed, rather than those of the 

 compound itself. 



Elementary chemical reactions which according to the binary mode of viewing- 

 compounds were supposed to consist of dualistic processes involving sometimes 

 the assumption of forces (like predisposing affinity) of a purely metaphysical 

 character, were now explained as consisting of atomic displacements, or inter- 

 changes of a kind well known to be of common occurrence. Thus the evolution 

 of hydrogen by the action of zinc on aqueous hydric sulphate was supposed to be 



