ADDRESS. Ixxi 



would not wish me to do so if I could ; for we do not meet here to study 

 chemistry ; I conceive that we meet here for the purpose of considering what 

 this wondrous activity in our science means, what is the use of it, and, true 

 to our object as embodied in the name of this Association, to consider what 

 we can do to promote the Advancement of Science. I propose to lay before 

 you some facts bearing on each of these questions, and to submit to you some 

 considerations respecting them. 



In order to ascertain the meaning of the work which has been going on in 

 chemistry, it will, I think, be desirable for us to consider the leading ideas 

 which have been in the minds of chemists, and which guided their operations. 



Now, since the father of modern chemistry, the great Dalton, gave to che- 

 mists a firm hold of the idea of Atoms, their labours have been continually 

 guided by that fundamental idea, and have confirmed it by a knowledge of 

 more and more facts, while at the same time steadily adding to our know- 

 ledge of the properties of atoms. Every chemist who is investigating a new 

 compound takes for granted that it must consist of a great number of atom- 

 clusters (called by him molecules), all of them alike, and each molecule con- 

 sisting of a certain number of atoms of at least two kinds. One of his first 

 endeavours is to ascertain how many atoms of each kind there are in each 

 molecule of the compound. I must not attempt to describe to you the various 

 kinds of experiment which he performs for the purpose of getting this infor- 

 mation, how each experiment is carried out with the aid of delicate instru- 

 ments and ingenious contrivances found by long experience to enable him to 

 obtain the most trustworthy and accurate results ; but I want to draw your 

 attention to the reasoning by which he judges of the value of such experi- 

 ments when they agree among themselves, and to the meaning which he at- 

 taches to their result. 



If the result of his experiments does not nearly agree with any atomic for- 

 mula (that is, if no conceivable cluster of atoms of the kinds known to be in 

 the compound would on analysis give such results as those obtained), the 

 chemist feels sure that his experiments must have been faulty : either the 

 sample of substance which he worked upon contained foreign matter, or his 

 analyses were not made with due care. He sets to work again, and goes on 

 till he arrives at a result which is consistent with his knowledge of the com- 

 bining-properties of atoms. It is hardly necessary to say that even the best 

 experiment is liable to error, and that even a result obtained with the utmost 

 care cannot be expected to afford more than an approximation to the truth. 

 Every good analysis of a pure compound leads to results which approximate 

 to those required by the Atomic Theory ; and chemists trust so thoroughly 

 to the truth of that guide, that they correct the results of such analysis by the 

 aid of it. 



The chemical idea of atoms serves for two purposes : — 



1. It gives a clear and consistent explanation of an immense number of facts 

 discovered by experiment, and enables us to compare them with one another 

 and to classify them. 



2. It leads to the anticipation of new facts, by suggesting new compounds 

 which may be made ; at the same time it teaches us that no compounds 

 can exist with their constituents in any other than atomic proportions, 

 and that experiments which imply the existence of any such compounds are 

 faulty. 



We have the testimony of the great Bcrzelius to the flood of light which the 

 idea of atoms at once threw on the facts respecting combining proportions 

 which had been accumulated before it was made known ; and from that time 



/2 



