68 E. HAANEL ON THE APPLICATION OF 
Tt ought to be mentioned that other iodides, as those of potassium and sodium, give 
white coatings, but these are easily differentiated from those of copper, cadmium and zine, 
by additional tests presently to be described. 
It may be interesting to point out that the elements which give iodide coatings are 
found in three horizontal lines of Mendelejeff’s periodic system,and that each line is 
characterized by commencing with a light colour and ending with a dark brown. 
The volatility of the iodides seems also to increase in each horizontal line according 
to the progression of the atomic weights of the respective elements. Indium and gallium 
were not at my disposal. The former, I know, would give a yellowish coating, and, from 
the position of gallium in the series, it is reasonable to infer that it alone would not form 
an exception to the rule, that three lines of Mendelejefl’s periodic system commencing, 
respectively, with copper, silver and gold, give volatile iodide coatings. 
VI.— DIFFERENTIATION OF COATINGS. 
In order to distinguish between similar coatings, ammonia and yellow ammonium 
sulphide are used as reagents. The former is applied by holding the tablet, coating down, 
over the open mouth of a bottle containing strong ammonia, the latter by touching the 
coating with a glass rod moistened with the reagent. 
Distinction between the coatings of Lead and Tin. 
The tin coating, treated with ammonia, disappears at once ; lead coating with difficulty 
and only for a time. 
(Treated with ammonium sulphide, the tin coating changes to whitish-yellow, the 
lead coating to black). 
Distinction of Silver coating from those of Lead and Tin. 
Yellow coating of silver not affected by ammonia. Coating heated before blow-pipe, 
but not touched by flame, changes from faint greyish-yellow to bright canary-yellow. 
When touched with blowpipe flame, coating turns grey at the point touched. 
Distinction of Arsenic coating from that of Antimony. 
Reddish-orange coating of arsenic treated with ammonium sulphide becomes lemon- 
yellow; the orange-red coating of antimony treated in a similar manner changes to orange. 
Distinction of Tellurium coating from that of Bismuth. 
Brown coating of tellurium disappears at once when treated with ammonia; the 
brown coating of bismuth when treated in a similar manner changes to orange yellow, 
which after a time becomes cherry red. (This is shown on plate II, fig. 15.) 
Distinction of Zinc coating from other white coatings of Todides as far as examined. 
White coating of zine disappears at once when treated with ammonia; other white 
coatings of iodides are not affected. 
