ON A NATURAL SYSTEM 



Vitriolatcd ponderous earth, commonly called 

 f pat urn ponderofunii aerated lime *, lluoratcd 

 lime, impregnated with the acid of the lapis fin- 

 dcrofus f , are all faline earths, by the force of 

 compofition, and arc even without douht folu- 

 ble, though to what extent experience has not 

 yet determined. 



$ LII. Diflingui/bing Marks of Earths. 



Tit characters of earths are of the negative 

 kind. An earth is that fubftance, which is not 

 ibluble ; not fo heavy us metallic bodies, nor is 

 capable of combuflio.ii. Criteria fuch as theic 

 betray our ycry limited and imperfect know- 

 ledge. Croniledt indeed mentions another 

 mark, the malleability of earths ; but this ob- 

 fervaticn may be applied tofalts, phlogiflic Tub- 

 fiance, and the brittle metals. As to their 

 form not being changed by a red heat, the fame 

 can be fuid of the vitriolated vegetable alkali, of 

 metals that require a much greater degree of 

 heat for their fufion, and of other foffils. Any 

 cxpuniion of their bulk is fcarce perceptible to 

 the eye, though a red !ic::t i'i always lure ro pro- 

 ducc it, unlcis counteracled by the difllpation of 

 fume volatile matter, as in c!;iy, aerated Jijiie, 

 and other fubilanccs. 



S 



* vvi. ;. p. 26. 



j Vol. iii. p. 2:8. * 



