Legal Chemistry / A Guide to the Detection of Poisons, Examination of Tea, Stains, Etc., as Applied to Chemical Jurisprudence

by Alfred Naquet

In case the weapon has been wiped or exposed to moisture subsequent to its seizure, it is impossible to form any conclusion as to the date of its discharge, etc. It is therefore advisable, upon receiving the weapon, to carefully wrap the lock in a woollen cloth, and to close the barrel. The exterior of the gun is at first submitted to a careful examination, and notice taken of the approximate thickness of any existing rust spots. The fire-pan and adjacent portion of the barrel are also examined by aid of a magnifying glass, especial attention being given to the detection of traces of a moist and pulverulent incrustation of a greyish or blackish color, formed by the combustion of the gunpowder, and of crystals of sulphate of iron. If the weapon is loaded, the wad is withdrawn and the color of its cylindrical portion and of the powder, as well as the size of the ball or shot, noted.

This preliminary examination ended, the barrel and fire-pan are separately washed with distilled water, and the washings passed through filter paper which has previously been well washed, first with pure hydrochloric acid, then with distilled water. The filtrate is next divided into three portions, and these separately examined for: (1) sulphuric acid, by addition of chloride of barium; (2) for iron, by oxidizing the salts contained in the fluid with a few drops of nitric acid and adding a solution of ferrocyanide of potassium, the presence of iron being indicated by the formation of a blue coloration, or a blue precipitate; and (3) for sulphides, by means of a solution of subacetate of lead.

If a bluish-black incrustation is discovered on the fire-pan or on the neighboring portions of the barrel, and both rust and crystals of sulphate of iron are absent, and the washings, which were originally of a light-yellow color, assume a chocolate-brown coloration upon the addition of solution of subacetate of lead, the gun has been discharged within two hours at the longest.

If the incrustation possesses a lighter color and traces of iron have been detected in the washings, but neither rust nor crystals have been discovered on the barrel or fire-pan, the weapon has been discharged more than two, but less than twenty-four hours.

In case minute crystals of sulphate of iron and spots of rust are found, and the washings contain iron in a considerable quantity, the weapon has been discharged at least twenty-four hours, at the longest ten days.

If the quantity of rust found is considerable, but iron is no longer to be detected, the discharge of the gun occurred ten days, at the longest fifty days, previously.

If the weapon has been reloaded immediately after its discharge without having been previously washed, the portions of the wadding which have come in contact with the barrel will possess a greyish-black color during the first four days, the color gradually becoming lighter, until, at the fifteenth day, it turns grey and remains so permanently. In this case, the washings will contain sulphuric acid. The objection has been advanced to the last test that sulphuric acid might be discovered, even if the gun had not been discharged, if the paper of which the wadding was made contained plaster. M. Boutigny states, however, that this objection is untenable, if the wadding has not been moistened by the water introduced into the barrel.

In case the gun has been washed and dried before being reloaded, the cylindrical portion of the wadding possesses an ochre-yellow color up to the first or second day, assumes a decided red hue on the days following, and acquires a clear rusty color on the sixth day. During the fifth day the powder also possesses a reddish appearance, owing to an admixture of rust. Sulphuric acid is not present in the washings.

If the weapon has been reloaded immediately after being washed, the wadding possesses a greenish-yellow appearance for the first few hours, and subsequently acquires a reddish color, as in the preceding case.

If, finally, the barrel has been washed with turbid lime-water, rust is still to be found and the wadding possesses the color mentioned above. The following colorations are also to be observed in case the gun has not been washed, or has been dried near a fire: