G. Rose on QsmuiiWumfrom the Ural. 101 



It is otherwise manifest, that provided the fixed points on 

 which the system rests be capable of supplying resistance in 

 every possible direction, this parallelism is necessary to the 

 condition of a minimum amount of resistance. 



When, however, the resistances of the system are supplied 

 by the contact of surfaces capable of motion upon one another, 

 this condition of resistance in every possible direction is not 

 satisfied ; the possible directions of resistance at each point 

 lying within the surface of a right cone having that point for 

 its vertex, the normal to it for its axis, and a certain angle 

 dependent upon the friction of the surfaces in contact for its 

 vertical angle. 



In this general case, then, a parallelism of the resistances is 

 impossible, and we must determine the direction of each ac- 

 cording to the principles laid down in the preceding pages, 

 subject to such modifications as may be imposed by the na- 

 ture of the resisting surfaces and the other conditions of the 

 equilibrium of the system. The discussion of these is re- 

 served for a future Number of this Journal. 



XV. On the crystallized Compounds of Osmium a?id Iridium, 

 found in the Ural. By G. Rose.* 



Osmiridiumfrom r |^HE inclination of r and c, measured 

 Newransk. ■*• with the reflective goniometer, is 118° 



nearly. The faces r, c and g possess but little lustre. Cleav- 

 age parallel to c 9 but obtained with 

 difficulty. Colour tin white, rather 

 darker than native antimony. Lus- 

 tre metallic. Hardness equal to 

 that of quartz. Specific gravity 

 19*385, obtained by weighing 2*084 

 grammes of the mineral, selected with great care, in water, at 

 the temperature of 12°*3 Reaumur : a second experiment gave 

 19-471, the temperature of the water being 9° Reaumur. 

 Heated before the blowpipe on charcoal, it undergoes no 

 change, and does not give the slightest smell of osmium. It 

 smells slightly of osmium when melted with nitre in a matrass, 

 and forms a green mass after cooling. It is found in the 

 gold-sand of Newransk, 95 wersts north of Catharinenburg. 

 Platina occurs with it, but in much smaller quantities. It is 

 also found at Bilimbajewsk, Ryschtim, and many other places 

 in the Ural. 



Osmiridiumfrom NischneTagil. — The crystals of this variety 



* From Poggendorff's Annalcn, vol. xxix. Part III. 



