264 
Miscellany. 
at 32° F., and boils at 212° F, : the common acid boils at 218° F., and 
crystallizes at — 5° F. 
Test for Hydrocyanic acid. — Mr. Barry of London is of opinion that the 
nitrate of silver is the most delicate test for the presence of hydrocyanic 
acid, detecting the ten-thousandth part of a grain. The application is 
very simple. The suspected fluid is to be slightly acidulated by the ad- 
dition of acetic acid. If an excess of acid be present it is to be not quite 
neutralized by carbonate of soda. Two or three drops are then put in a 
watch glass, and immediately covered with a plate of glass, whose un- 
der surface, to the breadth of a pea, is moistened with a solution of ni- 
trate of silver, formed by dissolving one grain of lunar caustic in 100 
grains of distilled water. 
If the inverted drop of the silver solution retain its transparency, the 
absence of prussic acid is established, for had it been present, the test 
would in a few moments have become clouded by the formation of a white 
precipitate, an effect which indeed is almost instantaneous when the prus- 
sic acid is not excessively diluted. If, on the other hand, the precipitate 
appeared, the conclusion must not be drawn, that it is cyanuret of silver, 
until identified as such by two properties ; first, its speedy resolubility, 
as denoted by the cloudy drop becoming clear when placed over a vessel 
of caustic ammonia, in which respect it differs from the silver compounds 
of iodine and bromine ; and secondly, in retaining unchanged its pure 
white colour after exposure to the sun's rays or to a strong light. As 
this property essentially distinguishes it from the compound of silver with 
chlorine, it is important to establish it by a separate experiment, upon a 
somewhat larger portion of the precipitate, which should be obtained by 
candle light, by successively placing the inverted drop of nitrate of silver 
over renewed portions of the liquid in a saucer ; as soon as the precipitate 
separates into distinct and like particles, it is ready for exposure to the 
sun's rays. 
Another property which distinguishes the cyanide of silver from the 
chloride, is, that upon being ignited in an open glass tube, the cyanogen 
burns with a flame of the usual colour, leaving the metal pure, if suffi- 
ciently heated, a quality the more valuable as it famishes an index to the 
proportion of prussic acid it represents, which upon ordinary occasions 
may be estimated as equal to one-fourth the weight of the residual silver, 
Philos. Mag. 
