04 On the Cause of Chemical Pioporlions. [F>:b. 



Arsenic acid 34 100 



Oxide of lead 6f> 194-1 1 



100 



Now these 194"ll parts of oxide of load contain 13*878 parts of 

 oxygen. These, whether multiplied by two or three, give no 

 number which corresponds with the composition of arsenic acid as 

 above stated. 



Arseniate of Barytes. — When the solution of arseniate of soda 

 was mixed with that of muriate of barytes, no precipitate fell at 

 first; but the arseniate of barytes gradually deposited itself in the 

 state of small crystalline scales, which appeared quite insoluble in 

 water : 10 parts of this arseniate heated to redness, and afterwards 

 dissolved in nitric acid, produced with sulphuric acid poured into 

 the solution S'61)3 parts of sulphate of barytes. On repeating the 

 experiment, I obtained 8'65 parts of sulphate of barytes. Sup- 

 posing this last salt to contain 65-6 per cent, of barytes, arseniate 

 oi barytes is composed of 



Arsenic acid 42-974 100-00 



Barytes 57-026 132-70 



100-000 



According to the second experiment, 100 parts of arsenic acid 

 were combined with 131-2 parts of birrytes. 



Now 132-7 parts of barytes contain 13-89 parts of oxygen, and 

 131-2 contain 13-77 : ^o tliat the analysis of arseniate of lead lies 

 between these two numbers ; but we know that arsenic acid con- 

 tains more than twice that quantity of oxygen. It must, then, 

 contain three times 13'Si/, or 41-G7 per cent, of oxygen. 



1 thought the most accurate way of verifying this result would be 

 to decompose aisenious acid by means of sulphur in a small appa- 

 ratus of a determined weight. By finding the weight of tlie sul- 

 phurous acid gas disengaged, it would be easy to infer the quantity 

 «jf oxygen in arsenious acid. 1 mixed 5 parts of arsenious acid 

 with 20 parts of sulphur in a small retort, to the neck of whicii I 

 had fitted a narrow glass tube 3G inches in lengtli, to prevent any 

 of the sulphur from being mechanically carrii'd along with the sul- 

 phurous acid gas. I heated the retort till the disengagement of 

 sulp'iiurous acid gas was at an end, and the sulphuret of arsenic 

 began to sublime. The apparatus had lost 3-05 parts of its weiglit 

 jiy the disengagement of the sulphurous acid gas, in which there 

 ought to be 1-5185 part of oxygen. Hence 100 parts of arsenious 

 acid contain at least 30*37 pnrts of oxygen, and not, as has been 

 hitiierto believed, only 25 parts. Hence if arsenic acid contain* 

 IJ times as much oxygen as arsenious acid, it follows that it con- 

 tains at least 40 per cent, of oxygm. This agrees sufiicieutly with 



