LECTURE XXX. 



CHEMISTRY. 



EAETHS. 



WE are led next, by the arrangement we have 

 adopted, to consider earthy and stony substances. 

 The common definition of these substances is, 

 that they are bodies not soluble in water, not in- 

 flammable, and whose specific weight to that of 

 water is not more than four to one. This defi- 

 nition, like that of salts, is not so precise as to 

 be out of the reach of criticism, because there are 

 some earths perfectly soluble in water, and many 

 that are met with in the bowels of the earth that 

 afford the greatest probability that they have 

 been in a dissolved state. But the distinction, 

 though not strictly just, may be proper in a 

 looser sense; for there is a great difference of 

 solubility between earths and salts, for a few 

 grains of earth are sufficient to saturate a large 

 quantity of water, and even these dissolve less 

 perfectly than salts. There are two other cir- 

 cumstances, not included in the definition, that 

 make part of the idea of an earthy substance. 

 1 . A great degree of fixedness. 2. A disposition 

 to assume the form of a glassy concretion when 

 melted. By the first of these they are distin- 

 guished from all the other objects of chemistry, 

 for we cannot in general convert a pure earth 



