1820.] D>'. Tliomson on Arsenic I 85 



weidit amounting to 29 gr. ; but this cannot be allowing to the 

 loss of water, for the residual salt is no longer completely soluble 

 in water. The most accurate experiments which 1 have been 

 able to make give 28*31 of water in 50 of salt, or 56-G2 in 100 

 of salt. 



Fifty grains of the salt were dissolved in water, and precipi- 

 tated by nitrate of lead. The precipitate being washed, dried, 

 and heated to redness, weighed 49*8 gr. Now from the analysis 

 of arseniate of lead by Berzelius compared with my own experi- 

 ments upon the same salt, I consider it as established that it is 

 a compouird of 



Arsenic acid 7'2.5 



Protoxide of lead 14-00 



According to this statement, 49*8 gr. of arseniate of lead 

 contain 17 gr. of arsenic acid ; therefore, arseniate of soda must 

 be a compound of 



Arsenic acid 17-00 or 34-00 • 



Soda 4-69 9-38 



- Water 28-31 56-62 



50-00 100-00 



Now the weight of an integrant particle of soda is 4, and we 

 have 4-69 : 17 :: 4 : 14-5 ; so that the weight of acid in thi^ salt 

 bears to that of the base the proportion of 14-5 to 4. Therefore, 

 if we consider this salt as a compound of an atom of acid and an 

 atom of soda, the weight of an atom of arsenic acid will be 14-5. 



II. Arseniate of Potash. 



This salt has been long known, but has never been accurately 

 described. It was first formed by Macquer, who obtained it by 

 distilling in a retort a mixture of equal parts of nitre and arse- 

 nious acid.* This salt usually crystallizes in four-sided rectan- 

 gular prisms terminated by very short four-sided pyramids. I 

 have some very tine crystals of it obtained by spontaneous 

 crystallization. They are nearly three inches long, and of a 

 proportional thickness. In them the prisms gradually diminish 

 io size till they terminate in a point ; so thtit the crystals have 

 the appearance of consisting of enormously long octahedrons. 

 This salt has a saline and cooling taste, somewhat similar to that 

 of nitre. It is not in the least altered by exposure to the air. 

 Its specific gravity is 2-638, and it has a much firmer and more 

 solid texture than the crystals of arseniate of soda, indicating 

 obviously from its appearance that it contains much less water 

 of crystallization. It may be kept in a heat of 550~ for a consi- 

 tlerable time without melting or undergoing any remark-dble 



• Mem. Par, 1764, p. 223. 



