. 209. PHYSICAL PROPERTIES OF MINERALS. 307 



is contained in the works of HAUY. A glance at them will 

 suffice for enabling us to form an idea of their applicability. 



. 209. SPECIFIC GRAVITY. 



If we suppose the absolute weight of one of two 

 bodies, possessing the same volume, to be = 1 ; 

 the ratio of the absolute weight of the other to 

 this unit, is termed its Specific Gravity. 



The determination of the specific gravity depends upon 

 the comparison between absolute weights and volumes. 

 They cannot be instituted at all, or at least not to a suf- 

 ficient degree of accuracy, merely by sight or estimate. 

 We must avail ourselves of the assistance of appropriate 

 instruments, if we wish the determination to be of use. 



The instruments intended for ascertaining the specific 

 gravity of solid bodies, are the Hydrostatic Balance and 

 Nicholson's Araeometer. That of a liquid is determined 

 by weighing in it a solid body, whose specific gravity we 

 know, and which is not soluble in the liquid. Instruments 

 have likewise been constructed for this purpose. The de- 

 termination of the specific gravity of expansible fluids re- 

 quires very delicate operations, and instruments that are 

 not within the reach of every body. 



The arrangement of the two above mentioned instru- 

 ments, their use, and the whole process of taking the specific 

 gravity of bodies, are very generally known, or at least they 

 may be found described at large in every treatise on Natural 

 Philosophy. Each of them possesses particular advantages. 



The hydrostatic balance allows of a high degree of ac- 

 curacy, and is very convenient in its use. The instru- 

 ment being correct in itselfj and delicate as a common 

 balance, its delicacy as a hydrostatic one will depend upon 

 the thinness of the thread by which the vessel is suspended, 

 which bears the body immersed in the water. A human 

 hair is sufficiently strong for supporting a weight of three 

 hundred grains, and therefore very useful in taking the 



