1879.] Group of Metals known as the Platinum Series. 467 



Further, to carry out the purification, the blue powder (oxide of 

 iridium) is re- dissolved in aqua regia, evaporated to dryness, re-dis- 

 solved in water, and filtered. 



The dark-coloured solution thus obtained is slowly poured into a 

 concentrated solution of soda and mixed with hypochlorite of soda, 

 and should remain as a clear solution without any perceptible pre- 

 cipitate, and subjected in a distilling apparatus to a stream of chlorine 

 gas, should not show a trace of ruthenium when hydrochloric acid and 

 alcohol are introduced into the receiver. In this operation the 

 chlorine precipitates the greater part of the iridium in a state of blue 

 oxide, which after being collected, washed, and dried, is placed in a 

 porcelain or glass tube, and subjected to the combined action of 

 oxide of carbon and carbonic acid obtained by means of a mixture of 

 oxalic with sulphuric acid gently heated. 



The oxide of iridium is reduced by the action of the gas leaving 

 the oxide of iron intact, the mass is then heated to redness with 

 bi-sulphate of potash (which will take up the iron and any remaining 

 trace of rhodium) and after subjecting it to many washings with 

 distilled water, the residue is washed with chlorine water to remove 

 any trace of gold, and finally with hydrofluoric acid, in order to take 

 out any silica which might have been accidentally introduced with 

 the alkalies employed or have come off the vessels used. 



The iridium after calcination at a strong heat in a charcoal crucible, 

 is melted into an ingot, and after being broken up and boiled in 

 hydrochloric acid, to remove any possible trace of iron adhering to it 

 through the abrasion in breaking up, should possess if perfectly pure 

 a density of 22*39 ; but, as iridium prepared even with the utmost 

 care will still contain minute though almost inappreciable traces of 

 oxygen, ruthenium, rhodium, and possibly iron, the highest density I 

 have yet attained is 22 -38. 



Alloy of Iridio Platinum. 



This compound metal possesses physical properties of great value, 

 forming a beautiful example of the effect of a careful combination of 

 the opposite characteristics of its component parts. Thus, the 

 extreme softness and expansiveness of pure platinum and the brittle- 

 ness and excessive hardness of pure iridium, produce, by combination 

 in judicious proportions, a perfect and homogeneous alloy, possessing 

 the necessary mean of these properties to render it suitable for many 

 important purposes, amongst others that of the special object to be 

 attained to meet the requirements for an unalterable standard meted, 

 for which it is peculiarly adapted. 



In the manufacture of the prototype metres and the geodesique 

 rules (each 4 metres in length) ordered from my firm by the Comite 

 Internationale des Poids et Mesures, the Association Geodesique In- 



