>40 
nearly. It was probably by similar reasoning from a still 
more inaccurate 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 regarded 
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 to 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 12 times as heavy as hydrogen) he 
would obtain the atomic weight of sulphur as (12 x 1*28)— 1 
= 13*76, which number, standing half way between 14*4 as 
given in the first table and 13 as given in the second, points 
out the origin of the first relative weight of the ultimate 
particle of sulphur. So from sulphurous acid he would 
obtain a similar number, taking the specific gravity as ob- 
tained by him (Part II. 389) to be 2 '3, and remembering 
that this gas contains its own bulk of oxygen (p. 391), he 
obtained (2-3 - 1*12) x 12 = 14*16 for the atomic weight of 
sulphur. As however we do not possess the exact numbers 
of his specific gravity determinations, and as we do not 
exactly know what number he took at the time as re- 
presenting the relations between the densities of air and 
hydrogen (in 1803 he says that the relation of 1:0*077 is 
not correct, and that 2 V is nearer the truth), it is impossible 
to obtain the exact numbers for sulphur as given in the first 
/ 
table. 
In reviewing the experimental basis upon which Dalton 
