SECTION III, 1883. MO SM TRANS. Roy. Soc. CANADA. 
On the Application of Hydriodic Acid as a Blowpipe reagent.* 
By E. HaANer, Pu. D. 
(Read May 25th, 1883.) 
I. Some years ago it occurred to me that I might remove the difficulty experienced by 
students in distinguishing the oxyd-coating on charcoal of bismuth, from the similar one 
of lead by converting these coatings into iodides. The method adopted then was to touch 
the coatings with a drop of strong hydriodic acid, and direct the blowpipe flame upon the 
charcoal just in front of the moistened spot. The heat of the blowpipe flame volatilized 
the respective iodides, which were deposited again upon the cooler parts of the charcoal, at 
a greater distance from the assay. The iodide of lead gave a magnificent canary-yellow 
coating, the bismuth a chocolate-brown, cadmium and antimony, when treated in a simi- 
lar manner, a white and brick-red coating respectively. This has since been published. 
In the extension of this method to other substances, I found that other iodides of very 
characteristic colours were formed. Many of these were, however, altogether too volatile 
to be deposited satisfactorily on the charcoal, charcoal being too poor a conductor of heat 
to lower the temperature of the vapours of the iodides in question sufliciently to permit of 
their condensation and consequent deposition as coatings. 
In order to utilize to the fullest extent the value of hydriodic acid as a blowpipe 
re-agent, it became necessary to adopt a support which, on account of its better conductivity, 
would condense the various volatile iodides on its surface as coatings. The choice of the 
kind of support best suited was farther restricted by the following characteristics which a 
support, to prove entirely practical and satisfactory, should possess. 
1. It must be cheap and easily made. 
2. The surface of the support must be smooth and white, to bring out the colours of the 
coatings, uninfluenced by peculiarities of surface or admixture of tint of the support. 
3. It must resist the heat of the blowpipe flame. 
4. It must be of sufficiently porous texture to absorb the hydriodic acid, and supply it 
to the assay gradually and constantly during the progress of the operation. 
After some reflection and experimentation, I finally adopted plaster of Paris casts in the 
form of narrow, thin tablets, as the support, and found that it possessed the above-mentioned 
characteristics in an eminent degree. 
I] —PREPARATION OF TABLETS. 
The tablets are made by mixing plaster of Paris with water to a thin paste, and dis- 
tributing it over a large glass plate to the thickness of one-eighth of an inch. While yet 
moist and before the plaster has fully set, the surface is grooved with a knife, guided by a 

* With a supplementary note on the spectroscopic examination of the colours of coatings obtained by this 
method, by A. P. Coleman, Ph.D., communicated by Dr. E. Haanel. (See page 72). 
Sec. III, 1883. 9. 
