ADDRESS. Ixxiii 



Although anticipating my answer to these questions, you will probably be 

 surprised to hear the actual result which I have to record, a result so won- 

 derful that the more I think of it the more I marvel at it. Not only did 

 these various theories contain nothing at variance with the atomic theory ; 

 they were found to be natural and necessary developments of it, and to serve for 

 its application to a variety of phenomena which were unknown to its founder. 



Among the improvements of our knowledge of atoms which have taken 

 place, I ought to mention the better evaluations of the relative weight 

 of atoms of different kinds, which have been made since Dalton's time. 

 More accurate experiments than those which were then on record have 

 shown us that certain atoms are a little heavier or lighter than was then 

 believed, and the work of perfecting our observations is constantly going 

 on with the aid of better instruments and methods of operation. But, 

 apart from these special corrections, a more sweeping change has taken place, 

 not in consequence of more accurate experiments interpreted in the usual 

 way, but in consequence of a more comprehensive view of the best experi- 

 mental results which had been obtained, and a more consistent interpreta- 

 tion of them. Thus the atomic weight of carbon had been fixed at 6 by 

 Dumas's admirable experiments ; and it was quite conceivable that a still 

 more perfect determination might slightly increase or diminish this number. 

 But those who introduced the more sweeping change asserted in substance 

 that two of these supposed atoms, whatever may be the precise weight of 

 each, always are together and never separate from one another; and they 

 accordingly applied the term atom to that indivisible mass of carbon weighing 

 twice as much as a carbon atom had been supposed to weigh. So also with 

 regard to other elements, it has been shown that many atoms are really 

 twice as heavy as had been supposed, according to the original interpretation 

 of the best experiments. This change was brought about by what I may be 

 permitted to call the operation of stock-taking. Dalton first took stock of 

 our quantitative facts in a business-like manner ; but the amount and variety 

 of our chemical stock increased so enormously after his time, that the second 

 stock-taking absorbed the labours of several men for a good many years. 

 They were men of different countries and very various turns of mind ; but, 

 as I mentioned just now, they found no other fundamental idea to work 

 with than Dalton's ; and the result of their labo\irs has been to confirm the 

 truth of that idea and to extend greatly its application. 



One of the results of our endeavours to classify substances according to their 

 natural resemblances has been the discovery of distinct family relationships 

 among atoms, each family being distinguished by definite characteristics. 

 Now, among the properties which thus characterize particular families of 

 atoms, there is one of which the knowledge gradually worked out by the 

 labours of an immense number of investigators must be admitted to consti- 

 tute one of the most important additions ever made to our knowledge of these 

 little masses. 



I will endeavour to explain it to you by a simple example. An atom of 

 chlorine is able to combine with one atom of hydrogen or one atom of potas- 

 sium ; but it cannot combine with two atoms. An atom of oxygen, on the 

 other hand, can combine with two atoms of hydrogen or with two atoms of 

 potassium, or with one atom of hydrogen and one of potassium ; but we 

 cannot get it in combination with one atom of hydrogen or of potassium 

 solely. 



Again, an atom of nitrogen is known in combination with three atoms of 

 hydrogen ; while an atom of carbon combines with four of hydrogen. Other 



