39^ Dr Christison on the Detection of Arsenic in Fluids. 
ment a glass tube, closed at one end, open at the other, about 
three inches long, and varying from a fourth to an eighth of an 
inch diameter, according to the bulk of the material, which should 
not fill above two-fourths of an inch of the tube. The best 
mode of applying heat is by the alkohol lamp, as recommended 
by Mr Phillips. u The true arsenical crust is known by the 
following physical character. Its outer surface next the tube 
exactly resembles highly polished steel. Its inner surface, 
(which is best seen by scratching the tube with a file at the lower 
margin of the crust, and snapping it across), is precisely like 
the fracture of fine steel, if the quantity is considerable ; if it is 
minute, it has a dull bluish-grey appearance, but, before a mi- 
croscope of four or five powers, appears brilliant and crystal- 
line, like the fracture of steel. Occasionally, when very minute 
in quantity, it appears botryoidal, and not brilliant, even before 
the microscope : in that case, the part of the tube to which it is 
attached, should be coarsely powdered, and heated anew in a 
tube of less diameter. It is scarcely possible for any one to 
mistake these characters, particularly if he has ever seen an ar- 
senical crust before ; but, to prevent all possibility of error, the 
analysis may be concluded with the following experiment: The 
part of the tube to which the crust is attached being broken 
into fragments, is to be left for some hours in a watch-glass, con- 
taining a dilute solution of the ammoniacal sulphate of copper, 
and -covered to prevent evaporation. In four or five hours, the 
metallic crust will become grass-green ; or, if very minute, it 
will be discoloured, and a brilliant grass-green crust formed on 
the surface of the liquor. The simple evaporation of the fluid 
will cause the formation of a crust on its surface, though no 
arsenic be immersed in it. But in that case its colour is pale- 
blue V' The author concludes by stating, that the evidence thus 
procured is quite unimpeachable ; that the process is the most 
convenient yet proposed ; that it is probably applicable to all 
cases without exception, -as be has found it to answer with the 
most complicated fluids lie comd select, namely, biotli, tea, with 
cream and sugar, coffee similarly made, porter, Port-wine and 
* If portions of the crust are simply exposed to the air, they soon acquire a 
greyish-black colour on the surface ; a character alsc indicative of metallic arsenic. 
— Edit. 
