739 
1908-9.] The Atomic Weight of Platinum. 
In one case it was found that — 
Platinum boat and sponge cooled in air weighed 4*20520 
„ „ „ and weighed in vacuum 4*20516 
As we wish to know the amount of silver required for the precipitation, 
as well as the amount of silver chloride formed, two portions of silver are 
carefully weighed out, one for the precipitation of the chlorine in the 
hydrochloric acid, the other for the precipitation of the chlorine in the 
potassium chloride. A slight excess of silver, amounting to two or three 
milligrams, is used in each case. The silver is now dissolved in nitric acid, 
taking every care to prevent any of the silver being carried away mechani- 
cally with the escaping vapours. 
All the solutions are diluted to a low concentration before mixing, and 
the silver nitrate is added very slowly and with constant agitation to the 
chloride solutions. When precipitation is complete, the silver chloride is 
shaken up vigorously with the solution for perhaps half an hour, and then 
allowed to stand for at least ten hours before filtering. Actinic light is of 
course excluded entirely from the silver chloride. 
The solutions were filtered on platinum Gooch crucibles, using a good 
grade of Italian asbestos as a mat. Needless to say that the asbestos had 
been boiled in nitric and hydrochloric acids, and had been thoroughly 
washed. After washing, the precipitates were dried first for one hour at 
100°, and then for three hours at 150° in an electric drying oven. 
A platinum crucible, almost equal in weight to the crucibles used in filter- 
ing, and containing about the same amount of asbestos as one employs for a 
filtering mat, was always used as a tare when weighing the Gooch crucibles. 
The moisture which, as shown by Richards and Wells,* is always 
retained by silver chloride, even when dried at 150°, was determined by 
placing the pellet of silver chloride, after it had been weighed in the Gooch 
crucible, without any adhering asbestos, in a tared porcelain crucible ; 
weighing this combination, then heating until the mass was fused and 
weighing again. The loss in weight is applied as a correction to the weight 
of silver chloride previously determined. 
The small shreds of asbestos which were carried away from the mat 
while filtering, were collected on a small ashless filter and ignited ; and the 
weight of these was applied as another correction to the weight of silver 
chloride. Obviously this is opposite in sign to the previous correction. 
Unfortunately silver chloride is appreciably soluble in water, and 
therefore the amount which has dissolved in the water used in washing 
* Jour. Am. Chew,. Soc., xxvii. 475 (1905). 
