nf Organic Bodies. 17 



be done, especially when it contains a large amount of silica, 

 which also separates on the concentration of the liquid. It is 

 therefore best to evaporate the whole of the liquid to dryness, 

 and to separate the phosphate of lime in the course of the ana- 

 lysis. 



The residue is moderately heated until its weight remains 

 constant. If it be heated too strongly, decomposition of the 

 carbonates contained in it may readily occur, as when silica 

 and phosphates are present carbonic acid is expelled, which 

 in fact partly occurs, according to Heintz*, during evapora- 

 tion. After determining the weight of the dry residue of the 

 aqueous solution, it is analysed as follows : — 



The quantity of carbonic acid is first estimated by means of 

 pure nitric acid in a suitable apparatus. This is obtained by 

 the loss in weight of the apparatus. If, during this operation, 

 the silica has become separated in the acid liquid, it is removed 

 by filtration, and the chlorine precipitated from the filtered 

 liquid by solution of nitrate of silver. The excess of silver is 

 removed from the liquid after the separation of the chloride 

 of silver by muriatic acid, andit is then evaporated to dryness in 

 a porcelain capsule. The dry residue is moistened with mu- 

 riatic acid and treated with water. Some silica usually re- 

 mains undissolved, which, when added to that previously ob- 

 tained, gives the whole amount contained in the aqueous 

 solution. 



The liquid separated from the silica is treated with ammo- 

 nia, by which the entire amount which has been taken up by 

 the water is precipitated. It is filtered, washed, dried, heated 

 to redness, and weighed. Its weight, when deducted from 

 that of the residue after evaporation to dryness, yields the 

 true weight of the aqueous extract. It is mixed with the car- 

 bonized mass which has been exhausted by water. 



The liquid is then treated with oxalic acid. In very ^evf 

 cases only is a precipitate of oxalate of lime thrown down, in 

 most cases it is not so. The oxalate of lime is determined as 

 carbonate by the ordinary method. 



The filtered liquid is then treated with a solution of chloride 

 of barium. The precipitate consists of phosphate, sulphate 

 and oxalate of baiyta. It is filtered and washed. Although 

 the oxalate of baryta is not very difficultly soluble, yet after a 

 little practice we can readily judge when the washing must be 

 discontinued. It is therefore advisable, on precipitating the 

 lime, not to add too large a quantity of oxalic acid, so as to 

 avoid obtaining a too copious precipitate of oxalate of baryta. 



The precipitate is treated with dilute muriatic acid ; sulphate 

 * PoggendorfTs Jnnalen, vol. Ixxii. p. 120. 

 Phil. Mag. S. 3. Vol. 35. No. 233. July 1 849. C 



