I 76 Account of Dr. IVolJaston's [Sept. 



and indefatigable chemist has somehow or other deceived himself, 

 and that the results which he has cb'ained are not correct. 



If we were to consider the result of my experiment as coming 

 nearer the truth than those of preceding experimenters, it would 

 follow from it that arsenic acid is composed of 



Arsenic 100 



Oxygen 52'4 



And supposing the oxygen in arsenious acid to that in arsenic acid 

 as two to three, which there can be no doubt is really the case, 

 arsenious acid must be composed of 



Arsenic 100 



Oxygen 3493 



Now this agrees very nearly with the determination of Thenard, 

 who found that 100 arsenious acid required 16 oxygen to be con- 

 verted into arsenic acid, and hence concluded that ic is composed of 

 arsenic 100 + 34 - 'JLH oxygen. The constituents of arsenic acid as 

 here stated coincide very nearly with the determination of Proust, 

 who found it composed of 100 arsenic + 52 - 905 oxygen. These 

 coincidences give me some confidence in the accuracy of my deter- 

 mination. Were we to put any confidence in it, an atom of arsenic 

 would weigh 5 '7-6, which is somewhat less than the weight 

 assigned in my original table. I cannot say that the evidence 

 adduced by Berzelius for the existence of two new oxides of arsenic 

 appears to me satisfactory. His reason for admitting the oxide sup- 

 posed to be obtained by subliming arsenic and calomel is little more 

 than analogy. At any rate, as the oxide was not obtained in a 

 separate state, its existence cannot be considered as put beyond 

 doubt. As to the supposed black oxide obtained by exposing 

 arsenic to the open air, for any thing that appears to the contrary, it 

 may be a mixture of metallic arsenic and white oxide of arsenic. 

 It appears to me at present, from all the facts with which I am 

 acquainted, that arsenic in its oxidation is similar to sulphur, phos- 

 phorus, iron, nickel, and cobalt. It combines with two proportions 

 of oxygen, bearing to each o'her the ratio of two to three. Whe- 

 ther a protoxide of arsenic exists does not at present appear, though 

 it is by no means unlikely. 



Article III. 



Account of Dr. WollastorCs Scale of Chemical Equivalents.* 

 This scale is of so much convenience to the practical chemist, 



* Dr. WollaHon's paper is published in the Philosophical Transactions for 

 i:-l !, part i. 



