CHARLES L. BRISTOL. D117 
Paris that combine all these conditions in an eminent 
degree. They are readily made by spreading a thickish 
paste of plaster over an oiled glass surface about one- 
eighth of an inch thick. While yet moist and before 
the plaster has fully set, the surface is cut with a knife 
guided by a ruler, so as to mark off tablets about 
four by one and three-quarters inches, though slightly 
smaller tablets may be used with as good results. When 
dry they easily separate and yield a glossy, white sur- 
face. 
The hydriodic acid is prepared by passing sulphuret- 
ted hydrogen through water containing finely divided 
iodine in suspension. The resulting clear liquid is the 
resulting acid dissolved in water containing more or 
less hydrogen sulphide. This latter, however, is no 
hindrance; in fact it is an advantage, since it retards 
the decomposition of the unstable acid and in no way 
interferes with the reaction. The instability of the acid 
is practically of little importance, says Haanel, because 
free iodine is readily reconverted by passing hydrogen 
sulphide into the solution. 
In manipulation, the assay is placed near one end of 
the tablet in a slight cavity, if necessary, and moist- 
ened with one or two drops of hydriodic acid, which is 
at once absorbed by the surrounding plaster. The assay 
is now gently treated with the oxidizing flame, and cer- 
tain oxides, chlorides, bromides and sulphides are thus 
decomposed by the hot vapor of the acid rising from 
the tablet around the assay; and any volatile iodides 
are deposited upon the tablet at greater or less distance, 
according to the degree of volatility. These coatings 
are in some cases very striking and characteristic, not to 
say beautiful.’ 
At this point I must introduce another important 
improvement in the use of iodine, made by Messrs. 
1 See Trans. Royal Soc. of Canada. 1883. 
LOH 
