. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . these perforations is conducted a glasstube with an external trumpet-shaped mouth, andthrough the other a rectangular glass tube providedwith a finely drawn point. So soon as the apparatusand reagents have themselves been found free fromarsenic by a blank experiment, small pieces of the sub-stance are placed inside the bottle, together with some 412 LABORATORY WORK. pure zinc, and the cork is inserted; through the verti-cal glas
. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . these perforations is conducted a glasstube with an external trumpet-shaped mouth, andthrough the other a rectangular glass tube providedwith a finely drawn point. So soon as the apparatusand reagents have themselves been found free fromarsenic by a blank experiment, small pieces of the sub-stance are placed inside the bottle, together with some 412 LABORATORY WORK. pure zinc, and the cork is inserted; through the verti-cal glass funnel some pure sulphuric acid is then runin, and as the result of an interchange between theacid and the zinc, hydrogen gas is liberated, and willescape at the small orifice in the finely drawn pointof the rectangular glass tube—where it can be hydrogen flame is a colourless one, but if anyarsenic be present it unites with the hydrogen formingarseniuretted hydrogen, which burns with a charac-teristic bluish flame. If a piece of clean cold por-celain (white) be brought against this flame, the steel-grey metallic mirror of arsenic is Fig. 97—Apparatus employed for Marshs test. Arsenic may exist in varying quantity in any material,^ from a fraction of a grain (of arsenious acid) to even80 grains per square yard—but since the smallest traceswill condemn the article containing it, there is no neces-sity here to enter upon the subject of a quantitativeanalysis. It may be pointed out, however, that it iscustomary to estimate the metal as arsenious acid,( white arsenic = AS2O3). Supposing, for instance, a certain quantity of a greenmaterial yields 0-05 gramme of Scheeles green, thisarsenite of copper (2 CuO,HO,As203) is found to have a ARSENIC IN WALL PAPERS, ETC. 4I3 combined atomic weight of 375, of which arsenious acid (AsaO,) forms 198 ; therefore i^ of the o-os s^ramme of 375arsenite of copper = arsenious acid. It is frequently useful to take
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