MEDICO-LEGAL CONTRIBUTIONS ON ARSENIC. 
441 
metallic arsenic will be formed upon the porcelain. Berzelius 
observed that if the tube was heated, the gas was decom¬ 
posed, and that metallic arsenic was deposited upon its walls. 
PfafF and Thompson made, independently of each other, the 
discovery that antimony formed a similar compound with 
hydrogen, and that it exhibited phenomena similar to those 
already noticed in the case of arseniuretted hydrogen. This 
last discovery destroyed confidence in Marshes test, until it 
was found that the two compounds could readily be distin¬ 
guished. 
Marsh’s apparatus, as originally proposed, was open to 
numerous objections; improvements have been made, how¬ 
ever, by which these have been very completely removed. 
The apparatus here described 1 have used with much ad¬ 
vantage, although somewhat more complicated than the form 
usually employed ; it fulfils its objects so perfectly, that the 
trouble of making it is amply compensated for by its effi¬ 
ciency. 
The generating vessel is a Wolfe’s bottle; its three necks 
are closed by corks, in which are accurately fitted a funnel- 
tube, a syphon-tube,—for drawing off the liquid when in 
large quantity, without admitting air and long suspending 
the operation,—and a bulbed tube filled with fragments of 
pumice-stone or charcoal moistened with sulphuric acid. A 
tube bent thrice at right angles connects the drying bulb 
■with the drying tube; this has in its lower part, immediately 
above the constriction, a little asbestos or cotton to prevent 
the mass above from falling down. The lower half of the 
tube is to be filled with caustic potash, in fragments the size 
of a pea; the upper half is nearly filled with fragments of 
chloride of calcium; finally, a little asbestos or cotton is 
placed above. 
The caustic potash serves two purposes,—that of drying the 
gas and removing any acid that may be present, which other¬ 
wise might prevent the formation of arsenical spots. Thus, if 
sulphuric acid were carried over, it would liberate chlorhydric 
acid from the chloride of calcium; and again,if a chlorhydric 
solution w’ere to be used in the apparatus (which is not to be 
recommended), the effect above mentioned might occur ; the 
potash removes all danger of this happening. The chloride 
of calcium serves simply to dry the gas. The mouth of the 
drying tube is closed with, a cork, through wdiich passes a 
short tube bent at right angles. The reduction tube ought 
to be of Bohemian glass,—this will not readily fuse, and, 
being free from lead, will not blacken w'hen heated ; it is 
connected with the mouth of the drying tube by a small bit 
