21 8 POISONS : THEIR EFFECTS AND DETECTION. [§ 2JI. 
In poisoning by bitter almonds no inflammatory change in the 
mucous membrane of the coats of the stomach would be anticipated, 
yet in one recorded case there seems to have been an eroded and inflamed 
patch. 
§ 271. Tests for Hydrocyanic Acid and Cyanide of Potassium.— 
1. The addition of silver nitrate to a solution containing prussic acid, 
or a soluble cyanide, 1 produces a precipitate of argentic cyanide. 100 
parts of argentic cyanide are composed of 80-60 Ag and 19-4 CN, 
equivalent to 20-1 HCN. It is a white anhydrous precipitate, soluble 
either in ammonia or in a solution of cyanide of potassium. It is soluble 
in hot dilute nitric acid, but separates on cooling. A particle of silver 
cyanide, moistened with strong ammonia, develops needles; silver 
chloride treated similarly, octahedral crystals. It is insoluble in water. 
Upon ignition it is decomposed into CN and metallic silver, mixed with 
a little paracyanide of silver. 
A very neat process for the identification of cyanide of silver is the 
following :—Place the perfectly dry cyanide in a closed or sealed tube, 
containing a few crystals of iodine. On heating slightly, iodide of 
cyanogen is sublimed in beautiful needles. These crystals again may 
be dissolved in a dilute solution of potash, a little ferrous sulphate 
added, and hydrochloric acid, and in this way Prussian blue produced. 
If the quantity to be tested is small, the vapour of the acid may be 
evolved in a very short test tube, the mouth of which is closed by the 
ordinary thin discs of microscopic glass, the under surface of which is 
moistened with a solution of nitrate of silver ; the resulting crystals 
of silver cyanide are very characteristic, and readily identified by the 
microscope. 
2. If, instead of silver nitrate, the disc be moistened with a solu¬ 
tion of sulphate of iron (to which has been added a little potash), and 
exposed to the vapour a short time, and then some dilute hydrochloric 
acid added, the moistened surface first becomes yellow, then green, lastly, 
and permanently, blue. No other blue compound of iron (with the 
exception of Prussian blue) is insoluble in dilute hydrochloric acid. 
3. A third, and perhaps the most delicate of all, is the so-called 
sulphur test. A yellow sulphide of ammonium, containing free sulphur, 
is prepared by saturating ammonia by SH 2 , first suspending in the fluid 
a little finely precipitated sulphur (or an old, ill-preserved solution of 
sulphide of ammonium may be used). Two watch-glasses are now taken ; 
in the one the fluid containing prussic acid is put, and the second (pre¬ 
viously moistened with the sulphide of ammonium described) is inverted 
over it. The glasses are conveniently placed for a few minutes in the 
water-oven ; the upper one is then removed, the moist surface evaporated 
1 In the case of testing in this way for the alkaline cyanides, the solution must 
contain a little free nitric acid. 
