1821.] Atomic Weight of various Metals and Acids. 129 



portion of mercury which had been sublimed, and had escaped, 

 notwithstanding the glass cover on the flask. 



I attempted to decompose calomel by means of a solution of 

 caustic potash in the way recommended by Mr. Donovan; but 

 did not obtain satisfactory results ; because I was obliged, in 

 order to dry the protoxide completely, to expose it to a tempera- 

 ture so high that it was partially reduced. If calomel be a com- 

 pound of 1 atom mercury + 1 atom chlorine (about which there 

 can be no doubt), it is evident that 29-5 grains of it, when 

 decomposed by caustic potash,, ought to give 26 grains of prot- 

 oxide. The quantity which I obtained in repeated trials varied 

 somewhat, but was in no case quite so high as 26 grains. 



V. Arsenious Acid, 



In an essay which is inserted in the Annals of Philosophy 

 (Second Series), vol. i. p. 13, I have shown that arsenic acid is 

 a compound of 



Arsenic 4-75 



Oxygen 3"00 



7-75 



and that it is a compound of one atom arsenic + three atoms 

 oxygen. I expressed a suspicion that arsenious acid was a com- 

 pound of one atom arsenic + two atoms oxygen; and promised to 

 endeavour to investigate the subject, and to lay the results before 

 the public as soon as I obtained any which appeared to me deci- 

 sive. It gives me pleasure to be now able to fulfil that promise. 



When arsenious acid and quicklime are mixed together in a 

 glass tube and heat applied, the acid is partly reduced to the 

 metallic state, and partly converted into arsenic acid. It occur- 

 red to me that by determining the quantity of arsenic acid 

 formed, and the quantity of arsenic sublimed, I should be able 

 to deduce the composition of arsenious acid with precision. 

 But the result of a good many trials made in this way was not 

 satisfactory. The arsenious acid was seldom decomposed com- 

 pletely. A portion of it sublimed along with the arsenic, and 

 prevented me from determining with certainty the quantity of 

 metallic arsenic evolved. I was in consequence obliged to 

 abandon this mode of experimenting. 



1 next tried whether it was possible to form a neutral arsenite. 

 A quantity of arsenious acid was boiled in a solution of caustic 

 potash ; but I found it impossible in this way to deprive the liquid 

 of the property of rendering cudbear paper purple. I next eva- 

 porated the whole to dryness, and by cautiously washing the 

 residual matter with distilled water, 1 succeeded in removing all 

 excess of alkali ; but as soon as this salt was dissolved in water, 

 crystals of arsenious acid deposited, and the liquid became alka- 

 line. I was obliged, therefore, to abandon the arsenites, as I 



New Series, vol. ii. k 



