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Test for the Presence of Blood

Gunning has discovered in acetate of zinc a reagent that precipitates the slightest traces of the coloring matter of blood from solutions, even where the liquids are so dilute as to be colorless. Blood washed from the hands in a pail of water can readily be detected in this way. The flocculent precipitate thrown down by acetate of zinc must be washed by decantation, and finally collected on a watchglass, and allowed to dry, when the microscope will readily reveal crystals if any blood be present. (See No. 6415.) 4394. Test for the Presence of a Free Acid. Dissolve chloride of silver in just sufficient ammonia to make a clear solution. If from 2 to 5 drops of syrup, made with 5 parts sugar to 4 of water, and shake the mixture.

If the liquid contain bile, a violet coloration is observed. Acetic acid, and those substances which are converted into sugar by sulphuric acid, may be substituted for sugar.