286 LIQUOR HYDRIODATIS ARSENICI ET HYJDRARGYRI. 
in accordance with the results of his formula, so as to contain the exact amount 
of ingredients therein specified. 
It is, however, admitted that the process is unnecessarily tedious, and the re¬ 
sult has not always been successful, even when it has been manipulated by 
chemists of considerable reputation. In some cases failure has been attributed 
to the want of attention to details, and more particularly to the long-continued 
incessant trituration until perfect union is effected of the double metallic iodides. 
Any abridgment of the time during which the trituration should be continued 
tends to leave the arsenic undissolved. 
Mr. Draper “states that unless the greatest care be taken to ensure the 
effectual combination of the iodine on heating the mixture, instead of its be¬ 
coming, as intended, nearly colourless, a great part of the arsenic remains un¬ 
dissolved ; and any continuance of ebullition only vaporizes the free iodine, as 
may be seen from the application of starch-paper. Thus, not only is its prepa¬ 
ration troublesome, but the strength of the product itself is liable to variation.” 
M. Soubeiran, in attempting to carry out Mr. Donovan’s process, found that 
the whole of the arsenic did not dissolve, and he was of the opinion that the 
ratio of iodine was insufficient; but Mr. Donovan attributed the failure to his 
not using the quantity of alcohol specified by him, thus leaving the mass dry 
before the elements were united. 
I am far from wishing, in any way, to disparage the efforts of the author in 
his desire to explain some of the causes of the failure, and obtain a perfect pre¬ 
paration ; still, it is difficult to reconcile the diversity of opinion that exists 
amongst chemists. 
First, as regards the exact colour the solution should have ; 
Secondly, as to whether Mr. Donovan’s process is really the best for effecting 
the desired result with certainty and success; and 
Thirdly, as to the relative strength of the solutions prepared by Mr. Dono¬ 
van’s and M. Soubeiran’s process. 
Mr. Heathfield states, “ When Mr. Donovan first made the solution, he found 
that it generally proved to be of a very pale yellow, and then only when seen 
in large quantities, sometimes being as pale as water.” In referring to Mr. 
Donovan’s original memoir, I find he states, “ It is scarcely worth while to ob¬ 
serve on the colour of the liquor of hydriodate of arsenic and mercury. I have 
described it as yellow ; Dr. Kane says it is colourless, and that it soon becomes 
yellow by the decomposition of hydriodic acid. During an extensive manufac¬ 
ture of it, I have never procured it colourless, except when the process failed. 
With me, it has always been of a light yellow hue from the first; and, so far 
from its becoming yellow, when its colour was purposely deepened by dissolv¬ 
ing In it an excess of iodine, a short exposure to light was sufficient to restore 
its original pale yellow tint. The liquid is also yellow when made by Sou¬ 
beiran’s process.* Dr. Pereira states that the solution is of a pale yellow colour 
with a green tinge.” 
With regard to the difference in the strength of Donovan’s and Soubeiran’s 
solutions, Mr. Donovan states thus :—“ The quantities of the respective ingre¬ 
dients employed by me were— 
Grains. Grains. 
Arsenic . 6-08 + Iodine 30-24 Plisson. 
Mercury . 15-38 + Iodine 19-38 Gay-Lussac. 
Total Iodine .... 49-62 
* I should infer that on the addition of a few grains of iodine to the solution when exposed 
to light, the water becomes decomposed, its hydrogen uniting with the iodine, forming hy- 
dnodic acid. I believe it is a well-known fact that a solution of chlorine exposed to light be¬ 
comes converted into hydrochloric acid. 
