LIQUOR HYDRIODATIS ARSENICI ET HYDRARGYRI. 
287 
I employ 50 grains, for the sake of round numbers. M. Soubeiran, in prefer¬ 
ence to my method, recommends iodide of arsenic and biniodide of mercury to 
be dissolved in boiling water. He finds them dissolve perfectly. Yet between 
his method and mine there is no difference in the ratio of materials used; for, 
calculating from the data contained in his memoir, the quantity of iodine ne¬ 
cessary for the above quantities of arsenic and mercury would be as follows :— 
Grains. Grains. 
Arsenic . 6’08 + Iodine 31-70 
Mercury . 15-38 + Iodine 19-12 
Total Iodine .... 50 82 
Which is, within a grain and one-fifth, the same as I employ ; and the only dif¬ 
ference is, that he uses the two iodides ready formed, while I form the same ex¬ 
temporaneously. If his method succeeds, so must mine.” 
Of the five processes referred to by Mr. Heathfield, there is one well deserv¬ 
ing of especial attention, viz. that of M. Soubeiran, in which he proposes to 
unite the biniodide of mercury with the teriodide of arsenic,—two definite che¬ 
mical combinations,—and thus form the solution. 
Mr. Heathfield, in commenting upon M. Soubeiran’s process, states that it is 
open to this objection, that the biniodide of mercury and the teriodide of arsenic 
vary in the proportion of moisture they contain. This difficulty, however, cau 
be easily overcome ; if the biniodide of mercury has been prepared by precipi¬ 
tation, and has been imperfectly dried, the moisture can readily be removed by 
sublimation ; but in the case of the sublimation of iodide of arsenic, much care 
is required to prevent the formation of arsenious acid in the sublimate. But, 
if dry sublimed iodine be fused with finely-divided metallic arsenic, and the 
mass afterwards be finely levigated and then re-fused, any doubt as to the pre¬ 
sence of moisture would be at once removed. Having prepared large quantities 
of the solution by both processes, and after carefully reviewing the two methods, 
and forming a comparison between them both, I should decidedly give the pre¬ 
ference to M. Soubeiran’s, on the grounds of its easy manipulation and absolute 
certainty of success, the two metallic iodides being perfectly soluble in boiling 
water, and the two sometimes differing from each other by one grain and one- 
fifth of iodine in eight ounces of the solution, the proportion of arsenic and 
mercury remaining exactly the same. I would, therefore, venture to recommend 
for practical adoption the following proportions 
Sublimed Biniodide of Mercury . 172| grains. 
Teriodide of Arsenic.188^ ,, 
Distilled Water.40 ounces. 
The solution should measure exactly forty ounces, and should not give a blue 
colour when starch-paper is immersed in it. 
Mr. Douovau expressly states that the solution prepared by his method should 
be of a pale yellow colour. Mr. Heathfield, following Dr. Kane, states that it 
should be colourless. Now I think this discrepancy may be easily explained, 
for I find if the yellow solution, prepared either by Mr. Donovan’s or M. Sou¬ 
beiran’s process, be agitated, without the assistance of heat, with a few grains 
of finely-levigated metallic arsenic, they become at once permanently as pale 
and colourless as water ; hence it is quite possible that both these chemists may 
have had a slight excess of arsenic present. That the yellow colour is not due to 
the presence of free iodine may be readily proved by its not giving a blue colour 
with starch paper. 
