156 SCIENCE PROGRESS 



pounds of the same element. I re-produce his table of compounds 

 of chlorine : 



Weights of chlorine in the molecules, 

 Name of substance containing chlorine. referred to the weight of a semi-molecule of 



hydrogen as unity. 



Hydrochloric acid 



Chlorine 



Mercuric chloride 

 Arsenious chloride 

 Stannous chloride 

 Stannic chloride . 



35'5 = 35*5 



71 = 2 X 35-5 

 71 = 2 X 35-5 



. 106-5 = 3 X 35-5 

 71 = 2 X 35-5 



. 142 = 4 X 35-5 

 etc. etc. etc. = « x 3S"5 



Atomic weight of chlorine — 35*5 



Having presented similar orderly arrangements of facts 

 concerning gaseous compounds of each of several elements, 

 Cannizzaro arrives at a conception of the atom which is 

 perfectly lucid and contains in itself the method of determining 

 the relative weights of atoms. As true genius always does, 

 Cannizzaro goes straight to the centre of the problem. He 

 grasped and held "the particular go of the thing," around 

 which two generations of chemists had ** hummed and buzzed " 

 for half a century. 



Here is Cannizzaro's statement of the law of atoms : 



" By comparing the different quantities of one and the same 

 element which are contained, either in the molecule of the 

 free element, or in the molecules of its compounds, the following 

 law stands out in relief: f/ie different iveights of one and the same 

 element contained in tJie various molecules are always ivhole 

 multiples of one quantity, ivliieh is justly called the atonal, because 

 it invai'iably oders the compounds zvithout division!^ 



At last, forty-seven years after Dalton had started chemists 

 on the path, the open clearing in the tangle of facts is reached ; 

 a survey can be taken of the wood ; the directions which new 

 paths should follow are clearly seen. Well might Lothar 

 Meyer declare that Cannizzaro had made the scales fall from 

 his eyes, and had replaced his doubts by a feeling of quiet 

 security. 



It was now easy to construct simple, self-consistent, in- 

 telligible formulas, which should express the compositions of 

 molecules in terms of their constituent atoms, for all gaseous 

 and gasifiable compounds. 



