ARSENIC. 
§ 767-] 
is a normal constituent of the organs named. On the other hand, Ziemke 
( Vrtljschr. f. ger. Med., 1902) examined twelve human thyroids by Gautier’s 
method, and in only one found a trace of arsenic. Hodelmoser (Zeit. f. 
physiol. Chem., 1900) also examined various human thyroids, with a 
negative result. If arsenic were a normal constituent, there could be 
no exception, and therefore its presence in the cases in which it has been 
found must be adventitious. It must also be pointed out that the 
parts usually examined for poison by the analyst—stomach, liver, kidney, 
intestines,—investigated with the greatest care, are in the majority of 
cases absolutely arsenic-free save in cases of poisoning by arsenic. 
§ 767. Detection of Arsenic. —The analyst may have to identify 
arsenic in substance, in solution, in alloys, in wall-papers, in earth, in 
organic arseno-compounds, and in various animal, fatty, resinous, or 
other organic matters. 
Arsenious Acid in Substance. —The general characters of arsenious 
acid have been already described, and are themselves so marked as to 
be unmistakable. The following are the most conclusive tests — 
1. A small fragment placed in the subliming cell (p. 261), and 
heated to about the temperature of 137*7° (286° F.), at once sublimes in 
the form of an amorphous powder, if the upper glass disc is cool; but 
if heated (as it should be) to nearly the same temperature as the lower, 
characteristic crystals are obtained, remarkable for their brilliancy and 
permanency, and almost always distinct and separate. The prevailing 
form is the regular octahedron, but the rhombic dodecahedron, the 
rectangular prism, superimposed crystals, half crystals, deep triangular 
plates like tetrahedra, and irregular and confused forms, all occasionally 
occur. 
2. A beautiful and well-known test is that of Berzelius :—A small 
hard-glass tube is taken, and the closed end drawn out to the size of a 
knitting-needle. Within the extreme point of this fine part is placed 
the fragment (which may be no more than a milligramme) and a 
splinter of charcoal, fine enough to enter freely the narrow part, as 
a 
shown in the figure. The portion of the tube containing the charcoal 
(c) is first heated until it glows, and then the extreme end ; if arsenic 
is present, a mirror-like coating is easily obtained in the broader 
portion of the tube ( d). That this coating is really arsenical can be 
established by the behaviour of metallic crusts of arsenic towards 
solvents (as given at p. 593). The portion of the tube containing the 
crust may also be broken up, put in a very short, wide test-tube (the 
mouth of which is occupied by a circle of thin microscopic glass) and 
