550 poisons : their effects and detection. [§729. 
is important to identify, if possible, any green matters found in the 
stomach. In any case, it must be remembered that although rhubarb 
has been eaten for centuries, and every schoolboy has occasionally 
chewed small portions of sorrel, no poisoning has resulted from these 
practices. When oxalic acid has been taken into the stomach, it will 
invariably be found partly in combination with lime, soda, ammonia, 
etc., and partly free; or if such antidotes as chalk have been administered, 
it may be wholly combined. Vomiting is nearly always present, and 
valuable evidence of oxalic acid may be obtained from stains on sheets, 
carpets, etc. In a case of probably suicidal poisoning, the senior author 
found no oxalic acid in the contents of the stomach, but some was 
detected in the copious vomit which had stained the bed-clothes. The 
urine also contained a great excess of oxalate of lime—a circumstance 
of little value taken by itself, but confirmatory with other evidence. 
If a liquid is strongly acid, oxalic acid may be separated by dialysis 
from organic matters, and the clear fluid thus obtained precipitated by 
sulphate of lime, the oxalate of lime being identified by its microscopic 
form and other characters. 
The usual general method for the separation of oxalic acid from 
organic substances or mixtures is the following :—Extract with boiling 
water, filter (which in some cases must be difficult or even impossible), 
and then precipitate with acetate of lead. The lead precipitate may 
contain, besides oxalate of lead, phosphate, chloride, sulphate, and 
various organic substances and acids. This is to be decomposed by 
sulphuretted hydrogen, and on filtering off the sulphide of lead, oxalic 
acid is to be tested for in the filtrate. This process can only be 
adopted with advantage in a few cases, and is by no means to be recom¬ 
mended as generally applicable. The best general method, and one 
which ensures the separation of oxalic acid, whether present as a free 
acid, as an alkaline or a calcic oxalate, is perhaps the following :—The 
substance or fluid under examination is digested with hydrochloric acid 
until a fluid capable of filtration is obtained ; the free acid is neutralised 
by ammonia in very slight excess, and permitted to deposit, and the 
fluid is then carefully decanted, and the deposit thrown on a filter. The 
filtrate is added to the decanted fluid, and precipitated with a slight 
excess of acetate of lime—this precipitate, like the first, being collected 
on a filter. The first precipitate contains all the oxalic acid which was 
in combination with lime ; the second, all that which was in the free 
condition. Both precipitates should be washed with acetic acid. The 
next step is to identify the precipitate which is supposed to be oxalate 
of lime. The precipitate is washed into a beaker, and dissolved with 
the aid of heat by adding, drop by drop, pure hydrochloric acid ; it is 
then reprecipitated by ammonia, and allowed to subside completely, 
which may take some time. The supernatant fluid is decanted, and the 
