272 PROF. H. E. ROSCOE ON DALTON's 



tained the composition of carbonic acid gas by heating a 

 given weight of carbon with oxide of lead, and he came to 

 the conclusion that this gas contained 28 parts by weight 

 of carbon to 72 parts by weight of oxygen. Now Dalton 

 not only was acquainted with the properties and composi- 

 tion of carbonic acid, but he was aware that Cruikshank 

 had shown, in 1800, that the only other known compound 

 of carbon and oxygen, carbonic oxide gas, yields its own 

 bulk of carbonic acid when mixed with oxygen and burnt ; 

 and also that Desormes* analyzed both these gases, finding 

 carbonic oxide to contain 44 of carbon to 56 of oxygen, 

 whilst carbonic acid contained, to 44 of carbon, 112 of 

 oxygen, being just double of that in the carbonic oxide. 

 Dalton adds, "this most striking circumstance seems to 

 have wholly escaped their notice." Hence Dalton assumed 

 that one atom of carbon is united, in the case of carbonic 

 oxide, with one atom of oxygen, whilst carbonic acid pos- 

 sessed the more complicated composition, and contains two 

 atoms of oxygen to one of carbon. Now, if carbonic acid 

 contains carbon and oxygen in the proportion of 28 to 72, 

 carbonic oxide must contain half as much oxygen, viz. 

 28 of carbon to 36 of oxygen, and assuming that the 

 atomic weight of oxygen is 5*5, that of carbon must be 



28 x V? 



y" = 4*3. Having thus arrived at the number 4*3 



as the first atomic weight of carbon, it is easy to see why 

 Dalton gave 6*3 as the atomic weight of carburetted hy- 

 drogen from stagnant water, and 5-3 as that of olefiant 

 gas. The one represents one atom of carbon to 2 of hy- 

 drogen; the other, one of carbon to one of hydrogen; 

 or, olefiant gas contains to equal quantities of carbon 

 only half as much hydrogen as marsh-gas. This conclu- 

 sion doubtless expressed the results of Darton's own expe- 

 riments upon these two gases, which were made, as we 



* Ami. de Chimie, t. 39. p. 38. 



