1820.] Dr. Thomson on Arsenic, 87 



precipitate which fell. The Hquid was now concentrated by 

 evaporation, and a quantity of alcohol poured into it. A preci- 

 pitate fell consisting of rhomboidal crystals of a blue colour. 

 Chenevix found the constituents of this salt as follows : 



Acid 14-50 



Black oxide of copper 12*83 



Water 8-82 



Now the number which represents the weight of an integrant 

 particle of black oxide of copper is 10. I think, therefore, that 

 it is not unlikely (if we suppose an atom of arsenic acid to weigh 

 14"5) that this salt is a compound of 1 atom of arsenic acid 

 s= 14-5 + i atom of black oxide of copper =10 + 8 atoms of 

 water = 9 ; so that this arseniate rather favours the opinion that 

 an atom of arsenic acid weighs 14*5. 



The second of Mr. Chenevix's varieties of arseniate of copper 

 occurs in thin hexahedral plates, has a fine emerald green colour, 

 and a specific gravity of 2-548. Its constituents were found 

 to be: 



Arsenic acid 14-50 



Black oxide of copper 13*50 



Water 6-07 



This (reckoning the weight of the atoms as before) is a com- 

 pound of 1 atom acid + iJ- oxide + 6 atoms water, making 



Acid 14-50 



Oxide 13-33 



Water 6*75 



The third and fourth varieties of Chenevix obviously agree in 

 their composition ; or if there be any difference, it consists in 

 the water. They agree in colour (green) and in specific gravity, 

 viz. 4-280. How far the crystalline forms can be reconciled 

 remains still to be determined. The constituents of these two 

 varieties, according to the analysis of Chenevix, are as follows : 



They obviously consist each of 1 atom of acid united to 2-|- 

 atoms of oxide; while the first contains 9 atoms of water 

 = 10*125, and the second, 7 atoms water = 7*805. 



Mr. Chenevix's first variety occurs crystaUized in obtuse octa- 

 hedrons. Its colour varies from blue to green, and its specific 

 gravity is 2*881. Its constituents, according to Mr. Chenevix's 

 analysis, are as follows : 



Acid 14*5 



Oxide 50*0 



Water 35*7 



