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



It is not possible to ascertain the exact grounds upon 

 which Dalton gave the number 7*2 for phosphorus; its 

 juxtaposition, however, in the Table, to phosphuretted 

 hydrogen shows that it was probably an analysis or a 

 density -determination of this gas which led him to the 

 atomic weight 7*2, under the supposition that this gas (like 

 ammonia) consisted of one atom of each of its components. 

 In the second Table, published in 1808, Dalton gives the 

 number 9 as that of the relative weight of the phosphorus 

 atom ; and we are able to trace the origin of this latter 

 number, although that of 7*2 is lost to us. On p. 460, 

 part ii. of his 'New System/ Dalton states that he found 100 

 cubic inches of phosphuretted hydrogen to weigh 26 grains, 

 the same bulk of hydrogen weighing 2*5 grains. Hence 



26 — • 2* c 

 — ; — - = 9*4 gives the atomic weight of phosphorus. It 



2 5 

 was probably by similar reasoning from a still more in- 

 accurate experiment than this one that he obtained the 

 number 7*2. 



Sulphur, which stands in the first Table (of 1803) at 14*4, 

 was altered in the list published in the ' New System ' to 

 13. These numbers were derived from a consideration (1) 

 of the composition of sulphuretted hydrogen, which he re- 

 garded as a compound of one atom of sulphur with one of 

 hydrogen, and (2) of that of sulphurous acid, which he 

 supposed to contain one atom of sulphur to two of oxygen. 

 Dalton knew that the first of these compounds contained 

 its own volume of hydrogen ; and he determined its specific 

 gravity, so that by deducting from the weight of one 

 volume of the gas that of one volume of hydrogen he would 

 obtain the weight of the atom of sulphur compared with 

 hydrogen as the unit. The specific gravity he obtained 

 was about 1*23 (corresponding nearly, he says, p. 451, to 

 Thenard's number 1*23). Hence (as he believed air to be 

 twelve times as heavy as hydrogen) he would obtain the 



