382

ELEMENTS OF MATERIA MEDICA.

arsenious acid, and the garlictube. To obtain solutions ofFig. 62. & Fig. 61.

(a) A syphon tube.

( b ) Stop-cock.

(c) Wooden block.

(d) The pillar.

(ee) Caoutchouc slips, to fasten thetube to the pillar.

(f) Place of mica or glass.

Fig. 62 .—(</) Small glass bucket.

Fig. 63.

odour can be detected at either end of thethe acid, let the frame successively playbeneath three or four drops of water placedon the under side of a plate of mica; thenapply the liquid tests for arsenic before men-tioned (rierapath, Med. Gaz. xviii.889).

Various forms of apparatus may be usedfor this experiment. That employed byMr. Marsh is a simple glass tube, bentlike a syphon (fig. 61). A bit of glass rodis dropped into the shorter leg, then apiece of clean sheet zinc: the stop-cockand jet are afterwards to be inserted. Thesuspected liquid, mixed with the diluteacid before mentioned, is to be thenpoured into the long leg. Effervescenceis produced, and after allowing the air tobe expelled, the stop-cock is to be closed,and when a sufficient accumulation of gashas taken place, it is again to be opened,and the gas ignited. Where the matterto be examined is very small in quantity,Mr. Marsh puts the suspected liquid, theacid, and the zinc, in a little glass bucket(fig. 62), attached to the stop-cock by aplatinum wire, and then introduces it intothe short leg of the syphon, previouslyfilled with common water. A modifica-tion (fig. 63) of Mr. Marsh’s apparatus issupplied with two bulbs, one in each legof the instrument, and presents some ad-vantages over the simple syphon tube:thus it enables us to collect a larger quan-tity of gas, while the bulb assists in check-ing the frothing by breaking the bubbles.But the simplest, cheapest, and oltcn themost useful form of apparatus, is a two-ounce wide-mouthed phial, with a corkperforated by a glass tube or tobacco-pipe,as in fig. 64, p. 383, annexed. It presentsthis great advantage, that we can employ afresh apparatus for every experiment, andthus avoid all possibility of contamina-tion from arsenical liquids used in pre-vious experiments.

The impediments to the operation ofMarsh’s test are, organic liquids (as porter,soup, contents of the stomach, &c.), whichoccasion great frothing, and choke up thejet. The means of obviating this are,greasing or oiling the interior of the shortleg of the apparatus, putting a layer of