338 On some Combinations of Platinum. 



tinuin, as tests for gelatine; and I think I may venture to con- 

 clude, that the sulphate is a test of superior delicacy, and more 

 certain in its operation. Thus, in cases where the gelatine was 

 in very miiuite quantity, or in a very diluted state, when no ef- 

 fect was produced by strong infusions of oak-bark, nutgalls, or 

 catechu, there was an immediate precipitate on adding sulphate 

 of platiinim. In instances also, when the (juantity of gelatine 

 was too minute to be readily detected by simply adding the sul- 

 phate, the effect was immediately produced on boiling the fluid. 



The effects of sulphate of platinum on solutions of the dif- 

 ferent varieties of gelatine, as isinglass, glue, and size, appear 

 to be preciselv similar, and the precipitates obtained in such 

 cases seem to be uniform in their properties and composition ; 

 nor are they affected by the presence of any of the mineral acids 

 in excess. The operation of astringent infusions, as oak-bark, 

 nutgalls, and catechu, on solutions of the different varieties of 

 gelatine, is not uniform. According to Sir H. Davy, catechu 

 contains a much larger quantity of the tanning principle than 

 oak-bark ; vet I found that an infusion of it produced no pre^ 

 cipitate in solutions of size, of different degrees of concentration. 

 The size I employed was such as paper-hangers use; it ha"d 

 been recently prepared, and was, previous to its being dissolved 

 in water, in the form of a tremulous jelly. The sulphate of 

 platinum occasions, after a short time, a brown precipitate in 

 astringent infusions ; but this substance I have not examined. 



7. On a gray Oxide of P lat'mum , 



In the course of my experiments to ascertain the composition 

 of fulminating platinum, I treated it with nitrous acid, and thus 

 procured, as I have elsewhere stated, a gray oxide of platinum, 

 which has not vet been described. It may be obtained by adding 

 strong nitrous acid to fulminating platinum, boiling it to dryness, 

 and exposing the dry mass to a heat just below redness, so as to 

 expel all the nitrous acid. The oxide of platinum remains. It 

 is to be fuielv pulverized, and boiled, first in pure water, and 

 then in a weak solution of caustic alkali to separate the last 

 portions of acid, which adhere with great tenacity to it, It is 

 now to be vvell washed, and dried at a heat not exceeding that of 

 Vjoiling mercury. I have usually made the experiment in a 

 platinum crucilV.e on a hot sand-bath. The oxide thus prepared 

 exhibits the following properties. 



8. Properties and Conipoiition of the gray Oxide of Platinum, 

 Its colour is dark iron gray. It has the metallic lustre. It 

 IS snfiiciently hard to cut brass, which it polishes, and when the 

 polished surface is rubbed a little with tlie oxide, a delicate coat- 

 ing 



