PHYSICAL PROPERTIES OF MINERALS. 



121 



treatise. They can easily be learned by recurring to most of the 

 works on Natural Philosophy, where they are more appropriately 

 described, with every necessary detail. The instruments for deter- 

 mining the specific gravity of solid minerals, are the Hydrostatic 

 Balance and Nicholson's Jlrceometer. 



The Hydrostatic Balance consists of a pair of scales, sufficiently 

 delicate to be turned with the 100th of a grain, when loaded with 

 300 or 400 grs. The scales should be set upon a stand, and pro- 

 vided with an accurate set of weights, from 100 grs. down to the 

 20th of a grain. A hook is affixed, under one of the pans, to which 

 is attached the thread or filament, intended to give support to the 

 fragment whose specific gravity is to be ascertained. The vessel 

 which holds the water should be a glass jar, seven or eight inches 

 deep ; and a thermometer should be provided for adjusting the tem- 

 perature of the water. Its delicacy after all will depend upon the 

 thinness of the thread by which the mineral is suspended in the 

 water. A human hair is sufficiently strong for supporting a weight 

 of three hundred grains, and is therefore, in most cases, admirably 

 adapted to the purpose. 



Nicholson's jlrcsometer, Fig. 151, is a 

 hollow cylinder MN, made of while iron; 

 the stem Ir is a wire of brass, which sup- 

 porte the little cup A and the larger pan C. 

 This stem is marked, towards its middle, 

 by a slight impression b, made with a file. 

 From the lower part, is suspended the 

 leaden.bucket E. The weight of the in- 

 strument is such, that when we plunge it 

 into water, it swims in a vertical posture, 

 with the mark b somewhat elevated above 

 the surface of the water. In employing 

 this instrument, the same attention is re- 

 quisite, as respects the purity and tem- 

 perature of the water, as in the use of 

 the Hydrostatic Balance. Having intro- 

 duced the instrument into a tall glass jar 

 of this water, we load the pan C (placed 

 on A) with weights, until the Araeometer 

 sinks so as to make the mark b on the 

 stem exactly coincide with the surface of 



the water. The amount of weight required for this is marked upon 

 the cup, for future use, and is called the balance weight of the in- 



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