HYDRIODIC ACID AS A BLOWPIPE REAGENT. 69 
Distinction of Cadmium coating from all other white coatings of Iodides. 
White coating of cadmium treated with ammonium sulphide changes to orange. 
Distinction of Copper coating from all other white coatings of lodides. 
Copper coating when touched with reducing flame imparts to it a magnificent emerald- 
green colour. 
Copper, in presence of any other coating, is readily distinguished, even when present in 
but very small quantity, by the green colour imparted to reducing flame when the coating 
is touched with it. The green flame is also seen to play over the assay during the opera- 
tion of forming the coating, if the assay contains copper. The reaction is very sensitive. 
VII.—IDENTIFICATION OF ONE OR MORE ELEMENTS CAPABLE OF GIVING IODIDE COATINGS IN 
PRESENCE OF EACH OTHER. 
On account of the difference in the colours and the degree of volatility of the iodides, 
it Is, in many instances, not difficult, if the assay contain more than one of the elements 
capable of giving iodide coatings, to determine each of them satisfactorily at one single 
operation. 
Three cases may arise, which are important for such determination : 
1. It may happen that the elements in question, on account of the difference in the 
degree of their chemical affinity for the iodine, are eliminated successively. In this case the 
iodides will be deposited one after the other, and may thus be recognized separately. 
2. The elements may be eliminated together, but be of different degrees of volatility.* 
In this case the coatings will overlap each other only for a distance ; beyond this distance 
we Shall then have the individual coating of the most volatile, while nearer the assay, a 
fringe of the coating or the individual coating of the least volatile. 
3. The elements may be eliminated together, and be either of the same or nearly the 
same degree of volatility. In this case the iodides are deposited together, producing a 
compound coating with a resultant colour, which may serve to indicate the components. 
I have made a number of experiments with minerals and mixtures to demonstrate 
that separations of certain elements, and hence their identification in presence of each 
other, are quite easily effected with the aid of hydriodic acid. 
Results. 
Proustite—The silver coating appears close to the assay, the more volatile arsenic coating 
at a greater distance. The arsenic coating is tinged slightly reddish by the small 
percentage of antimony present in the mineral. The arsenic is first eliminated. 
The silver is deposited after it and toward the close of the operation. 


* Norn.—In this statement it is assumed that the length of the coating, or the distance from the assay over 
which it is deposited, indicates the degree of volatility of the respective iodides. It is evident, however, that not 
alone the volatility, but also the specific heat of the vapour affects the length of the coating. The length of coatings 
of two iodides of the same degree of volatility will be directly as their specific heats. Since neither the w latility 
nor the specific heats of the vapours of the iodides have been accurately determined, and since for the purposes 
in hand it is unimportant, I may be allowed to continue throughout this paper the assumption that the longest 
coating will be given by the most volatile iodide. 
