ON A NATURAL SYSTEM 



and when they arc more intenfe, by a ferics of 

 gems, cut exprcfsly for this purpofc. But this 

 property indicates lefs the matter, and its mix- 

 ture, than the various exficcutions aritlng from 

 different circumflances, the fubtlety and cohc- 

 fion of particles, dcniity, and iuch like. Soft 

 clay dried gradually, and afterwards expofcd to 

 an cncreafing lire for ievcral hours, until it i* 

 brought to a white heat, becomes harder and 

 harder, and is at length capable, like a Hint of 

 finking fparks from ilecl. In all this procefs, 

 however, tlie matter is no otherwife alfeded 

 than by a contraction of its bulk, which is di 

 miiiiihcd about one half. 



5 xxix. Specif c Gravity. 



SPECIFIC gravity is determined by the hydro- 

 (latic balance, which properly indicates nothing 

 ' elfe than the deiiitty or quantity of matter in a 

 given volume. A knowledge of this property 

 is of confidcrable utility, elpecially in the exa- 

 mination of metals, whether pure, or of known 

 mixture ; but with refpedl to other fofllls, the 

 diilcrence is fo very trifiling, that their nature 

 and compofition can fcarcely ever be this way 

 -ifcertained. 



XXX. 



