414 
LECTURES ON THE BRITISH PHARMACOPOEIA. 
commercial potash or pearlash by treating it with a small quantity of cold water, 
which dissolves the carbonate of potash and chloride of potassium, and leaves the 
greater part of the sulphate of potash which commercial potashes always contain. 
The solution thus formed is afterwards evaporated until the salt, while con¬ 
stantly stirred, becomes dry in the pan. It constitutes, as is well known, a dry 
granular salt, which, if exposed for any length of time to the air, attracts water, 
and becomes first moist and afterwards liquid. If it be well dried in its prepa¬ 
ration, and if it be carefully excluded from the atmosphere, it can be kept and 
used without any great difficulty ; and although it is not absolutely pure, it an¬ 
swers every purpose required in medicine. 
Besides this so-called salt of tartar, there is a pure carbonate of potash, which 
used to be, and still might be more correctly, called salt of tartar, because it is 
made from cream of tartar, or from bicarbonate of potash, by the application of 
heat; and this is usually kept in the anhydrous state for the delicate purposes 
of chemical analysis. 
Now, the carbonate of potash of the British Pharmacopceia is neither the 
salt of tartar of commerce nor the pure carbonate of potash of analytical che¬ 
mists, nor any other form of carbonate of potash to be found anywhere else than 
in chemical books. This new carbonate of potash is represented in the descrip¬ 
tion given of it as containing two atoms of water. This is indicated by the che¬ 
mical formula (K0,C0 2 ,2IIO) ; and it is further indicated by the quantity of 
water it is said to lose on the application of heat—namely twenty-one per cent., 
and by its neutralizing power when treated with the volumetric solution of ox¬ 
alic acid. Salt of tartar, on the other hand, contains only about an atom and a 
half of water, which is equal to 16 per cent., instead of 21 per cent. The diffe¬ 
rence then is, that the Pharmacopoeia salt contains half an atom of water more 
than is present in the salt of commerce. You may think this a very trifling 
difference ; but in effect it is an important difference. It just constitutes the 
difference between a convenient and a most inconvenient product. It represents 
also the difference between the composition of^crystallized carbonate of potash, 
as given in chemical books, and that of the dry, although still hydrated, carbo¬ 
nate as obtained by a practical operation, and in a state in which it is fit for 
use. The books are right in representing crystallized carbonate of potash as it 
is represented in the British Pharmacopoeia; but to obtain the salt in that state 
involves considerable difficulty. It can, however, be obtained in fine large, 
transparent prisms, which are so deliquescent that it is as difficult to keep as it is 
to obtain them. 
For potassa bicarbonas and soda bicarhonas , processes are given which, if it 
were desirable to make these preparations on a small scale, are quite unexcep¬ 
tionable. The processes are in operation before you, and the apparatus used is 
similar to that described in the Edinburgh and also in the Dublin Pharma¬ 
copoeias ; in fact, the process appears to be almost literally that of the Dublin 
College as given in their Pharmacopoeia of 1850. 
Magnesia and magnesia carbonas are changed from light to heavy, and pro¬ 
cesses are given which are perfectly efficient for the production of these varieties 
of magnesia. 
It is important to observe that, in future, when magnesia or carbonate of 
magnesia is ordered, the heavy variety is to be used; not the comparatively 
heavy, but the heaviest of the sort. 
Metallic preparations. —Amongst the metallic preparations we have a very 
good process for emetic tartar, now called 
Antimonium tartaratum , which consists in mixing oxide of antimony and 
cream of tartar into a paste with water, and allowing the mixture to stand for 
twenty-four hours; then adding more water, boiling for a quarter of an hour, 
filtering the solution, and setting it to crystallize. 
