[Footnote A:  When the paper is freshly prepared, and as long as it remains in good condition, a drop of diluted acetic acid put on the paper with a glass rod produces no coloration.  In process of time it will become brownish, when treated with the acid, especially if it has been exposed to sunlight.  It is then not fit for use.]

In Germany zinc-iodide starch paper is used, which is considered to be more sensitive than potassium iodide.

Standard Tint Paper.—­A solution of caramel in water is made of such concentration that when diluted one hundred times (10 c.c. made up to 1 litre) the tint of this diluted solution equals the tint produced by the Nessler test in 100 c.c. water containing .000075 grm. of ammonia, or .00023505 grm.  AmCl.  With this caramel solution lines are drawn on strips of white filter paper (previously well washed with distilled water, to remove traces of bleaching matter, and dried) by means of a quill pen.  When the marks thus produced are dry, the paper is cut into pieces of the same size as the test paper previously described, in such a way that each piece has a brown line across it near the middle of its length, and only such strips are preserved in which the brown line has a breadth varying from 1\2 mm. to 1 mm. (1/50 of an inch to 1/25 of an inch).

Testing Dynamite, Blasting Gelatine, and Gelatine Dynamite.—­Nitro-glycerine preparations, from which the nitro-glycerine can be extracted in the manner described below, must satisfy the following test, otherwise they will not be considered as manufactured with “thoroughly purified nitro-glycerine,” viz., fifteen minutes at 160 deg.  F. (72 deg.  C.).

Apparatus required.—­A funnel 2 inches across (d), a cylindrical measure divided into grains (e), Fig. 47.

Mode of Operation.—­About 300 (19.4 grms.) to 400 grains (26 grms.) of dynamite (b), finely divided, are placed in the funnel, which is loosely plugged by freshly ignited asbestos (a).  The surface is smoothed by means of a flat-headed glass rod or stopper, and some clean washed and dried kieselguhr (c) is spread over it to the depth of about 1/8 inch.  Water is then poured on from a wash bottle, and when the first portion has been soaked up more is added; this is repeated until sufficient nitro-glycerine has collected in the graduated measure (e).  If any water should have passed through, it must be removed from the nitro-glycerine by filter paper, or the nitro-glycerine may be filtered.

[Illustration:  FIG. 47.—­APPARATUS FOR SEPARATING THE NlTRO-GLYCERINE FROM DYNAMITE.]

[Illustration:  FIG. 48.—­TEST TUBE ARRANGED FOR HEAT TEST.]