824 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[April 19, 1873. 
perfection of the present mode of preparing this 
compound, that they have sought to obtain a better 
preparation by the double decomposition of a mer¬ 
curous salt—such as the acetate or the nitrate—and 
an alkaline iodide, such as iodide of potassium. 
But, as is well known, these mercurous salts are not 
soluble in water excejfi in presence of excess of acid ; 
and if such solutions be treated with iodide of potas¬ 
sium, the free acid displacing a small quantity of 
iodine produces a mixture of red iodide of mercury 
and metallic mercury, which contaminates the preci¬ 
pitate. Of course the mercuric iodide is always easily 
separated from such a mixture; but such is not the 
case with respect to the metallic mercury which ac¬ 
companies the green iodide through all the reactions 
to which it may be submitted. 
In order to prepare the green iodide by double de¬ 
composition, the problem consists in finding a mer¬ 
curous salt very soluble in water and neutral to test- 
paper. This I believe I have met with in the double 
pyrophosphate of soda and mercurous acetate, a salt 
which is included in the catalogue of double pyro¬ 
phosphates which have been described by Persoz* 
and Pahl.f It is known, in fact, that the pyrophos¬ 
phate of soda possesses the property of forming, with 
many salts and metallic oxides, definite compounds 
generally very soluble in water, Now I have found 
that with chemically pure pyrophosphate of soda 
mercurous acetate forms a compound which crys¬ 
tallizes in fine needles, changing after long exposure 
to air, but very soluble, without decomposition, in 
water. This salt, dissolved in a sufficient quantity 
of water, mixed with an equally dilute solution of 
iodide of potassium, yields a fine yellowish green 
precipitate, having exactly the composition Hgl. As 
to the pyrophosphate of soda, it appears to take no 
part in the reaction unless it be to present the mer¬ 
curous acetate to the iodide of potassium in a state 
of solution. 
To prepare the pyrophosphate of soda and mer¬ 
curous acetate, 60 grams of pure crystallized pyro¬ 
phosphate of soda are dissolved in 300 grams of 
warm distilled water. After the solution becomes 
cool, 30 grams of mercurous acetate are added, and 
the mixture is left to react during several hours at 
ordinary temperature, assisted by occasional shak¬ 
ings. If the pyrophosphate of soda is chemically 
pure, the mercurous acetate is entirely dissolved, 
without the slightest decomposition; but such is 
not usually the case. During the action of the red 
heat upon the neutral phosphate of socta, to con¬ 
vert it into pyrophosphate, there appears to be a 
partial separation of the phosphoric acid from the 
soda, or a temperature too high and too long-con¬ 
tinued eliminates a little phosphoric acid, so as to 
leave in the product of calcination a little free alkali. 
Practically, the pyrophosphate of soda usually re¬ 
duces a small quantity of the mercurous acetate to a 
mercuric salt and metallic mercury. This fact has 
no other importance than slightly diminishing the 
yield of green iodide, and may be avoided in great 
part by care in choosing pyrophosphate of soda as 
pure as possible. 
The solution of pyrophosphate of soda and mer¬ 
curous acetate is filtered and added to another 
volume of distilled water. A solution of 30 grams 
of iodide of potassium in a litre of water is then 
* ‘ Journ. de Pharm. et de Chimie’ [31, vol. xii. p. 218. 
t ‘ Bull. Soc. Chim. de Paris,’ 1873, vol. xix. p. 115. 
poured in a little at a time, shaking after each 
addition. When the salts are perfectly pure, the 
precipitate produced is at first brownish green, then 
green, in which state it much resembles green oxide 
of chromium ; but when it settles at the bottom of 
the vessel it has a yellowish green tint, which would 
lead to the presumption that it is a polychromatic 
salt. Neither in the first nor in the last phase of the 
precipitation is there any iodine or mercury set free,, 
as is the case when mercurous nitrate and iodide of 
potassium is used. But should the solution of pyro¬ 
phosphate of soda and mercurous acetate contain, as 
is usually the case, a little mercuric acetate, then 
towards the end of the operation a small quantity of 
red iodide is seen colouring the liquid a pale red ; 
but it is easy to get rid of this by means of a slight 
excess of solution of iodide of potassium, which, thus 
diluted, does not decompose the green iodide of mer¬ 
cury. As a further precaution, the absence of mer¬ 
curic iodide may be ensured by washing the precipi¬ 
tate with warm concentrated alcohol. The green 
iodide, washed sufficiently by decantation and after¬ 
wards collected upon a filter, should be dried at a 
moderate heat, and sheltered from the light. 
The process described above is much more costly 
than that usually followed, but I consider the quality 
of the product obtained so superior as to remove any 
such objection. 
THE MICROSCOPE IN PHARMACY. 
BY HENRY POCKLINGTON. 
{Continuedfrom p. 762.) 
SiMARUBiE Cortex.— Not in the B.P., but suffi¬ 
ciently generally used to be admitted here. General 
structure loose and spongy ; medullary rays narrow 
and sinuous, or wider and straight; liber cells small, 
not numerous, and with thin walls ; stellate cells nu¬ 
merous, large, porous, isolated, and aggregated in 
groups of three to six, sometimes more. Such is a 
brief general description of the structure of this 
bark, and with a few remarks on the more important 
points will suffice. 
The first point that strikes one is the exceeding 
thinness of the walls of the whole of the cells ex¬ 
cepting the stellates, and the thickness and solidity 
of the walls of the stellate cells. The whole struc¬ 
ture, apart from these latter cells, is that of an endo¬ 
genous water plant, so far as regards shape and deli¬ 
cacy of the cells. The liber cells themselves are 
almost wholly unthickened, and short tubular cells 
with relatively large central cavities. With these 
are associated almost perfectly globose, thin cells,, 
containing prismatic, doubly refractive crystals, mi¬ 
nute, and easily mistaken for small air-bubbles. 
Exceptionally long liber cells occasionally occur, and 
are apparently very minutely porous. The existence 
of pores can only be inferred from certain dispersive 
optical phenomena; they are entirely invisible under 
as high a power as can be used upon these tissues- 
with success. 
The stellate cells have generally very large central 
cavities, the smaller cells being nearly filled ; they 
are very hard, minutely porous, and the successive 
depositions of sclerogen can only be seen with diffi¬ 
culty, in many instances, after the prolonged use of 
powerful reagents. 
It is not an easy bark to examine, and cannot be 
