Colour varies exceedingly, as does alſo the ſize of bodies. We cannot ſufficiently wonder at the violence done to nature by the ſtudied ſeparation of earths from ſtones. The conſequence is, that a ſtone of a certain ſize muſt conſtitute one genus, whilſt the ſame thing, reduced to powder, muſt be placed under another genus, which ſhall not be found even in the ſame claſs.
Hardness not unfrequently varies even in the ſame ſpecimen. Soft clay dries in the fire, and at length acquires the hardneſs of flint. Steatites (ſoap-rock) which may be ſcraped with the nail, and many other matters harden in the ſame manner, and that ſometimes without any notable loſs of weight; ſo that bodies paſs through every different degree of hardneſs, without any other change of their mixture.
Texture, and external form of the particles, may ſeem at firſt ſight to depend more upon the conſtituent parts; but a calcareous particle, globular or ſhapeleſs, is found, upon the moſt ſcrupulous examination, to poſſeſs the ſame properties as a piece of ſpar; and in another place I have clearly ſhewn, that the ſchirl-like, garnet-like, hyacinthine, twelve-ſided, and other figures, are not unfrequently formed by nature out of the ſame materials[[3]]. And if we are liable to deception where ſo great a difference in external forms exiſts, what can we expect from leſs conſtant external qualities?
Superficial characters are therefore inſufficient. They cannot even enable us to diſtinguiſh calcareous from other earths, for the efferveſcence with acids is a chemical mark, and happens, too, in matters of very different natures. To paſs over other inſtances, let him who is able diſtinguiſh the plumbum aeratum and plumbum phoſphoratum (§ 182. § 183.) by external appearances only!
But let us not altogether deſpiſe external characters: it is of moment to know and mark them well[[4]]. They frequently enable the accuſtomed eye without troubleſome trials to acquire a degree of certainty, which wants only a few ſelect experiments to confirm it. Sometimes alſo the uſe depends upon external properties, evident to our ſenſes, as the hardneſs, the colour, the pellucidity, &c. Theſe therefore may with propriety be joined to thoſe which point out the conſtituent principles.