Chap. 4.] Marks of Chemical Union. 21 



will be rendered in fome degree opake ; but if com- 

 mon fait, or blue or green vitriol is added to water, 

 the fluid will Hill remain perfectly tranfparent, though 

 tinged with the peculiar colour of the fait. The for- 

 mer therefore is termed a mixture, the latter zjolution. 

 When a fluid has received fo much of any foiid body 

 that it will not diflblve a particle more, it is faid to be 

 Jaturated. 



The marks of chemical combination in bodies have 

 been accurately defined by a correcl: and ingenious 

 philofopher. The firft is a fyecific gravity exceeding 

 that of the beavieft ingredients of the compound. Though 

 he properly obferves, it does not necefiarily follow, 

 that where fuch denfity is wanting a chemical union 

 does not exift ; fince the peculiar ftrufture of the com- 

 pound, which does not admit water into its vacuities, 

 may prevent this property from being remarked ; or a 

 quantity of water may enter into a competition natu- 

 turally heavier than water, and yet cannot be always 

 made fenfible. 



Secondly, Tranfparency is always a mark of cne- 

 mical combination. Such union, however, is alfo 

 fometimes confident with opacity, as that effect may 

 fometimes arife from a mere mechanical arrange- 

 ment of parts, from the interpofition of fome mat- 

 ter not properly combined, or from too great thick- 

 nefs. 



Thirdly, Cryftallization proves that the parts have 

 been very minutely divided, and in general com- 

 bined with the menftruum (or fluid in which the bo- 

 dies have been difiblved). Other fubftances, how- 

 ever, may fometimes intrude themfelves into the cry- 

 ftallized bodies, though not chemically combined with 

 them. 



Fourthly, A difficulty of diflblving the compound 

 C 3 body 



