FIRST TABLE OE ATOMIC WEIGHTS. 271 



all we know is, that it was mainly by the consideration of 

 the composition of certain simple gaseous compounds of 

 the elements that he arrived at his conclusions ; and, in 

 order that we may form some idea of the data he employed, 

 we must make use of the knowledge which chemists at that 

 time (1803-5) possessed concerning the composition of the 

 more simple compound gases. 



As I can find no record of any explanation of these early 

 numbers, I venture to bring the following attempt to trace 

 their origin before the Society to whom we owe their first 

 publication. 



The first point to ascertain, if possible, is, how Dalton 

 arrived at the relation between the atomic weights of hy- 

 drogen and oxygen given in the Table as i to 5*5 (but 

 altered to 7 in 1808). The composition, of water by weight 

 had been ascertained by the experiments of Cavendish and 

 Lavoisier to be represented by the numbers 1 5 of hydrogen 

 to 85 of oxygen ; and this result was generally accepted by 

 chemists at the time, amongst others doubtless by Dalton. 

 That in those early days Dalton had actually repeated or 

 confirmed these experiments appears improbable. At any 

 rate he formed the opinion that water was what he called 

 a binary compound, i. e. that it is made up of one atom of 

 oxygen and one atom of hydrogen combined together. 

 Hence, if he took the numbers 85 to 15 as giving the com- 

 position of water, the relation of H = 1 to O would be 1 to 

 5*6, or nearly that which he adopted. It does not appear 

 possible to explain why Dalton adopted 5*5 instead of 5*6 

 for oxygen; it may, perhaps, have been a mistake or a 

 misprint, as there are two evident mistakes in the Table, 

 viz. 13*7 for nitrous oxide instead of 13*9, and 9*3 for ni- 

 trous gas instead of 9*7. 



Let us next endeavour to ascertain how he obtained the 

 number 4*3 for carbone (altered to 5 in 1808 and to 5*4 

 later on). Lavoisier, in the autumn of 1783, had ascer- 



