7 16 POISONS : THEIR EFFECTS AND DETECTION. [§§ 92 I- 923 . 
soluble salt is converted into insoluble compounds and excreted with the 
fseces, it is, in any case where poisoning by iron is suspected, of more 
importance to examine chemically the faeces and the whole length of the 
alimentary canal, than even the contents of the stomach. In particular, 
any black material lying on the mucous membrane may be sulphide of 
iron mixed with mucus, etc., and should be detached, dissolved in a little 
hydrochloric acid, and the usual tests applied. 
In the criminal cases alluded to, there were iron stains on certain linen 
garments which acquired an importance, for, on dissolving by the aid of 
nitric acid, they gave the reactions of chlorine and iron. Any stains 
found should be cut out, steeped in water, and boiled. If no iron is dis¬ 
solved the stain should then be treated with dilute nitric acid, and the 
liquid tested with ferrocyanide of potash, etc. It need scarcely be 
observed that iron-mould is so common on shirts and any fabric capable 
of being washed, that great care must be exercised in drawing conclusions 
from insoluble deposits of the oxide. 
2. CHROMIUM. 
§ 921. The only salts of chromium of toxicological importance are 
the neutral chromate of potash, the bichromate of potash, and the 
chromate of lead. 
Neutral Chromate of Potash, Cr0 3 K 2 0 = 194-7, containing 51-5 per 
cent, of its weight of chromic anhydride, Cr0 3 .—This salt is in the 
form of citron-yellow rhombic crystals, easily soluble in water, but 
insoluble in alcohol. Its aqueous solution is precipitated yellow by lead 
acetate or basic acetate, the precipitate being insoluble in acetic acid. 
If chromate of potash in solution is tested with silver nitrate, the red 
chromate of silver is thrown down ; the precipitate is with difficulty 
soluble in dilute nitric acid. 
§ 922. Potassic Bichromate, Cr 2 0 6 K 2 0=295-2, containing 68-07 per 
cent, of its weight of chromic anhydride, Cr0 3 .—This salt is in beautiful 
large, red, transparent, four-sided tables ; it is anhydrous and fuses below 
redness. At a high temperature it is decomposed into green oxide of 
chromium and yellow chromate of potash. It is insoluble in alcohol, but 
readily soluble in water.- The solution gives the same precipitates with 
silver, lead, and barium as the neutral chromate. On digesting a solu¬ 
tion of the bichromate with sulphuric acid and alcohol, the solution 
becomes green from the formation of chromic oxide. 
§ 923. Neutral Lead Chromate, PbCr0 4 =323-5; composition in 100 
parts, PbO, 68-94 ; Cr0 3 , 31-06.—This is technically known as “ Chrome 
Yellow ,” and is obtained as a yellow precipitate whenever a solution of 
plumbic acetate is added to the solution of either potassic chromate or 
bichromate. By adding chrome yellow to fused potassic nitrate, “chrome 
