562 PROFESSOR DITTMAR AND MR JOHN M'ARTHUR ON 



instance, the analyst's factor for K 2 : Pt differs from the ratio 2 x 39*136 : 194*8, 

 the causes of this difference must be ascertained, and this necessarily involves 

 an inquiry into the true value for "Pt"; Stas' numbers for KC1, &c, can, 

 of course, be taken for granted. We accordingly did take up this inquiry, 

 and in this sense our investigation joins on to Seubert's, which, of course, 

 was welcome to us, as affording a most valuable basis for this branch of 

 our work. 



Before proceeding to detail our experiments, let us state that, unlike our 

 predecessors, we did not merely analyse our chloroplatinates, but in their 

 productions used exactly known quantities of standard solutions of chloro- 

 platinic acid and (for instance) chloride of potassium ; so that the weights of 

 platinum and alkaline chloride contained in the precipitated chloroplatinate 

 could be ascertained by determining the small quantities of these which passed 

 into the mother-liquor, and deducting them from the weights of platinum and 

 chloride of potassium employed in the precipitation. In the preparation and 

 application of these standard solutions (and of standard solutions generally) we 

 used a system which was introduced some years ago by one of us in connection 

 with a series of analyses of ocean water,* and which consists in this, that the 

 solutions are standardised both by volume and by weight. The volume-titre 

 serves only for calculating the number of cubic centimetres of solution 

 containing a predetermined quantity of reagent : the exact weight of the 

 latter is calculated from the weight of the measured off quantum and the 

 weight-titre. 



The necessary special reagents were prepared in the following manner : — 



Chloroplatinic Acid. 



For the preparation of this reagent, we utilised two supplies of what 

 had been sold to us as "pure platinum." One of these came from the 

 St Petersburg Works, through the kind agency of a St Petersburg gentleman, 

 who happened to be in Glasgow at the time; the other, from Messrs Johnson, 

 Matthey, & Co. of London. The St Petersburg metal, when examined, 

 turned out to be contaminated with iridium so largely that we presume a 

 mistake must have been made in the transmission or execution of our order. 

 To purify the metal we dissolved it in aqua regia, as far as possible,t and 

 subjected the crude chloroplatinic acid thus produced to the process of 

 Schneider, which is so fully described by Seubert in his memoir. The pure 

 chloroplatinate of ammonium obtained was reduced to metal by ignition, the 

 platinum washed with hydrochloric acid, then with water, dissolved in aqua 



* "Challenger Reports," Physics and Chemistry, vol. i., published by order of Her Majesty's 

 Stationery Office, &c. Adam and Charles Black (and others), 

 t A good deal of black iridium remained. 



