542 POISONS : THEIR EFFECTS AND DETECTION. [§ 716. 
sufficient quantity of the acid into a strong flask, clamp it by suitable 
connections to a mercury pump, and sublime in a vacuum ; in this way 
a sufficient quantity may be sublimed a little above 100°. It is well to 
remember, not only its low subliming temperature, but also that an 
aqueous solution, if kept at 100°, loses acid ; hence all evaporating or 
heating operations must not exceed 89°, or there will be some loss. The 
effect of heat is first to drive off water, then, if continued up to about 
190°, there is decomposition into carbon monoxide, carbon dioxide, 
water, and formic acid ; the two reactions occurring simultaneously— 
c 2 h 2 o 4 =co 2 +co+h 2 o. 
c 2 h 2 o 4 =co 2 +ch 2 o 2 . 
Heated with sulphuric acid to 110°, the following decomposition 
takes place :— 
h 2 c 2 o 4 =h 2 o+co 2 +co. 
Oxalic acid decomposes fluor spar, the phosphates of iron, silver, zinc, 
copper, and the arseniates of iron, silver, and copper. It may be used 
to separate the sulphides of iron and “manganese from the sulphides of 
zinc, cadmium, uranium, cobalt, mercury, and copper—dissolving the 
former, not the latter. Many minerals and other substances are also 
attacked by this acid. 
If a solution of oxalic acid in water is boiled with ammonio or sodio 
terchloride of gold (avoiding direct exposure to light) the gold is pre¬ 
cipitated’— 
2 AuC 1 3 +3H 2 C 2 0 4 =6C0 2 + 6HC1+Au 2 . 
When black oxide of manganese (free from carbonate) is mixed with 
an oxalate, and treated with dilute sulphuric acid, the oxalic acid is 
decomposed, and carbon dioxide evolved— 
Mn0 2 + H 2 C 2 0 4 + H 2 S0 4 = MnS0 4 + 2H 2 0 + 2C0 2 . 
A similar reaction occurs with permanganate of potash. 
If to a solution of oxalic acid, which may be neutralised with an 
alkali, or may contain free acetic acid, a solution of acetate of lime be 
added, oxalate of lime is thrown down. This salt, important from an 
analytical point of view, it will be well to describe. 
§ 716. Oxalate of Lime (CaC 2 0 4 H 2 0), 1 part = -863 crystallised 
oxalic acid. This is the salt which the analyst obtains for the quantita¬ 
tive estimation of lime or oxalic acid ; it is not identical with that occur¬ 
ring in the vegetable kingdom, the latter containing 3H 2 0. Oxalate 
of lime cannot be precipitated for quantitative purposes from solu¬ 
tions containing chromium, aluminium, or ferric salts, since somewhat 
soluble compounds are formed. It dissolves in solutions of magnesium 
