737 
1908-9.] The Atomic Weight of Platinum. 
platinate, and the removal of all hydrogen chloride, the water in the bulb 
A is carefully evaporated into the absorbers, and any residue left is 
cautiously heated. 
The platinum black has now to be washed free from potassium chloride, 
and this proved to be one of the most difficult parts of the whole analysis. 
The platinum and chloride were washed into a platinum dish, and here 
heated to boiling with successive portions of distilled water. As already 
pointed out by Seubert,* if the platinum has been reduced at the lowest 
possible temperature, it will be in so fine a condition as scarcely to be 
retained by the filter ; on the other hand we find that when heated to such 
a temperature that it clings together and filters readily, it parts with the 
potassium chloride so reluctantly that a great many treatments with the 
boiling water are necessary. If, again, this treatment is continued too long, 
the platinum becomes more or less colloidal and will not be retained by the 
filter. It was found that the presence of a little potassium nitrate in the 
wash water greatly retarded this formation of colloidal platinum, and 
permitted the boiling with water to be carried on to a much greater extent. 
When the platinum has been washed free from potassium chloride in 
this way, it is put in a tared platinum boat, and this is placed in the 
combustion tube of the apparatus in which it had been reduced. There it 
is heated in a stream of hydrogen to a red heat, which converts it entirely 
into the form of spongy platinum. After cooling in the hydrogen it is 
desiccated and weighed. This ignition and conversion of the platinum 
black into sponge in the atmosphere of hydrogen was thought to be 
necessary, in order to make certain that no oxidation of the platinum in 
this finely divided state took place. At higher temperatures, it seems 
likely that platinum even in its most coherent form is slowly oxidised, and 
in a finely divided condition this would probably take place at a much 
lower temperature. 
A second weighing of the platinum was made after it had been heated 
in the air, to the highest heat of the bunsen burner. These two weighings 
always gave identical results, as the following data will show : — - 
No. 1. After heating in hydrogen 1st weighing add to tare 0’85094 
„ „ in air to approximately 950 C. „ 0’85092 
No. 2. „ „ in hydrogen 1st weighing „ 0'61483 
„ „ in air to 950° C. „ „ 0’61483 
From an examination of the work already done on the occlusion of gases 
by the different forms of platinum, it appears by no means certain that an 
* Loc. cit. 
VOL. XXIX. 
47 
