ANNALS 



OP 



PHILOSOPHY. 



FEBRUARY, 1820. 



Article I. 



On Arsenic. By Thomas Thomson, M.D. F.R.S. 



In a short article on arsenic inserted in vol. xiv. of the Annals 

 of Philosophy, p. 466, I showed that the result of the most 

 careful experiments hitherto made gave the composition of the 

 two acids of arsenic as follows : 



Arsenious acid 4-75 arsenic + 1-5 oxygen 



Arsenic acid 4-75 +2-5 



or, if we consider an atom of arsenic to weigh 9-5, the compo- 

 sition of these acids may be stated as follows : 



Arsenious acid 9-5 arsenic + 3 oxygen 



Arsenic acid 9'5 + 5 



According to this last notion, arsenious acid is a compound of 

 1 atom arsenic + 3 atoms oxygen, and arsenic acid of 1 atom 

 arsenic + 5 atoms oxygen, the weight of an atom or integrant 

 particle of arsenious acid will be 12-5, and of arsenic acid, l4-5. 

 This mode of viewing the subject has the advantage of gettino- 

 rid of the fractional parts of oxygen, which make their appear^ 

 nnce when an atom of arsenic is reckoned to weigh 4-75, as I 

 have done in the last edition of my System of Chemistry, and 

 indeed, as has been hitherto done by every chemical writer 

 whose works I have had an opportunity of seeing. 



The best way of determining wliether 14-5 ''or 7-25 be the 

 number which represents the weight of an integral particle of 

 arsenic acid seems to be a careful analysis of the arseniates. 

 Cnfurtunately the greater number of tlxese salts being insoluble 



Vol. XV. N° II. F 



