552 POISONS : THEIR EFFECTS AND DETECTION. [§ 731 . 
dumb-bell-like bodies, compound octahedra, and small, flattened, bright 
discs, not unlike blood discs, are frequently seen. It may be usually 
identified under the field of the microscope by its insolubility in acetic 
acid, whilst the ammonium-mag. phosphate, as well as the carbonate of 
lime, are both soluble in that acid. From urates it is distinguished by 
its insolubility in warm water. A chemical method of separation is as 
follows :—The deposit is freed by subsidence as much as possible from 
urine, washed with hot water, and then dissolved in hydrochloric acid 
and filtered ; to the filtrate ammonia is added in excess. The precipitate 
may contain phosphates of iron, magnesia, lime, and oxalate of lime. 
On treatment of the precipitate by acetic acid, the phosphates of the 
alkaline earths (if present) dissolve ; the insoluble portion will be either 
phosphate of iron or oxalate of lime, or both. On igniting the residue 
in a platinum dish, any oxalate will be changed to carbonate, and the 
carbonate of lime may be titrated with d.n. HC1 acid and cochineal 
solution, and from the data thus obtained the oxalate estimated. The 
iron can be tested qualitatively in acid solution by ferrocyanide of 
potassium, or it can be determined by the ordinary methods. If the 
qualitative detection of oxalate of lime in the deposit is alone required, 
it is quite sufficient evidence should the portion insoluble in acetic acid, 
on ignition in a platinum dish, give a residue effervescing on the addition 
of an acid. 
§ 731. Estimation of Oxalic Acid. —Oxalic acid is estimated in the 
free state by direct weighing, or by titration either with alkali or by 
potassic permanganate, the latter being standardised by oxalic acid. If 
(as is commonly the case) oxalic acid is precipitated as oxalate of lime, 
the oxalate may be— 
(а) Dried at 100° and weighed directly, having the properties already 
described. 
(б) Titrated with dilute sulphuric acid and permanganate. 
(c) Ignited, and the resulting carbonate of lime weighed ; or dis¬ 
solved in standard acid and titrated back—1 part of calcic carbonate 
corresponds to 1*26 part of crystallised oxalic acid, or *90 part of 
H 2 C 2 0 4 ; similarly, 1 c.c. of standard acid equals -05 of calcic carbonate 
(or -063 of crystallised oxalic acid). 
( d ) The oxalate may be dissolved in the smallest possible amount of 
hydrochloric acid, and boiled with ammonio-chloride of gold, avoiding 
exposure to light; every part of gold precipitated corresponds to -961 
part of crystallised oxalic acid. 
(e) The oxalate may be placed in Geissler’s carbonic acid apparatus, 
with peroxide of manganese and diluted sulphuric acid. The weight of 
the gas which at the end of the operation has escaped, will have a 
definite relation to that of the oxalate, and if multiplied by 1*4318 will 
give the amount of crystallised oxalic acid. 
