1S14.] On the Cause of Chemical Proportions: 97 



It is easy to see (abstracting a little inaccuracy in the experiment) 

 that the acid in tliis salt is combined with twice as mucli base as in 

 the neutral ar<enite, and therefore must contain twice as much 

 oxygen as the oxide with which it is combined. 



I made some experiments, likewise, with the arsenites of barytes ; 

 hut they presented exactly the same difhcuhies as the borates of 

 barytes. 



From the preceding experiments the two acids of arsenic ought 

 to be composed as follows : — 



1. Arsenious Acidi 

 Arsenic from G775 to 68 100 



Oxygen from 32*25 to 32 ... . 47-626 to 46-926 



2. Arsenic Acid. 



Arsenic 58-33 to 58-7 100 



Oxygen 41-67 to 41-3 71*439 to 70-4 



These determinations in maximum and minimum are calculated 

 from tlie two analyses of neutral arseniate of barytes. 



It is evident that the number of volumes of oxygen contained ia 

 these acids must be either 2 : 3 or 4 : 6. The composition of the 

 neutral arscnites spealis very decidedly in favour of the last num- 

 bers ; but that of the neutral arseniates seems favourable to the 

 first : and so much the more, because, except the suboxide of 

 arsenic, we know no other oxides of that metal than the two acids : 

 but, on tlie other hand, the composition of the subarseniates does 

 not agree well with tlie supposition that arsenic acid is As + 3 O ; 

 for in that case (since the oxide of lead is P + 2 O) the subarseniate 



of lead would be P O + IJ- As O. Now the fraction \- is nowhere 

 else to be found in chemical compounds : but if arsenic acid be 



As + 6 O, the sul)ar<eniate in question will be As O + n P O. 

 These considerations alone would induce me to admit six volumes 

 of oxygen in arsenic acid ; but there are other proofs, more con- 

 vincing, of which 1 shall now give an account. 



M, Laugier has found that realgar, or native sulpluiret of arsenic, 

 when oxidated by nitromuriatic acid, and treated with muriate of 

 barytes, yields from 300 to 304 per cent, of sulphate of barytes. 

 As sulphate of Ijarytes contains 13-76 per cent, of sulphur, this 

 quantity is equivalent to 41-28 or 41-83 per cent, of sulphur in the 

 tulpliuret of arsenic. Hence 100 parts of arsenic combine with 

 71'.) or 7I'^'9 of sulphur : but this quantity is equal to tlie oxygen 

 in arsenic acid. Hence tliis sulph.uret is proportional to a degree 

 of oxidation of arsenic which contains half as much oxygen as the 

 acid in ic -. but this oxide is luikiiown. If It exist, arsenic acid 

 must contain 6 volumes of oxvgen ; otlierwisc this new oxide would 

 be As + 1^ O. 



As 1 was not in jiossession of any native realgar, I endeavoured 

 lo prepare it artiliclally by distiiling sulphur with arsenious acid. I 



Vol. 111. N^ H. G 



