88 A SYSTEM OF 



e. Black, La terrc noire^ at Montmartrc. 

 /. Violet, alfo from Mont mart re *. 



SECT. LXXIX. 



B. Indurated, Indurata. 



Is commonly uncluous to the touch, 

 and more or lefs difficult to be cut or turned, 

 in proportion to its different degrees of hard- 

 nefs •, is not diffufible in water, grows hard, 

 and is very refractory in the fire ; pounded 

 and mixed with water, it will not eafily co- 

 here in apafte : however, if it is managed with 

 care, it may be baked in the fire to a mafs, 

 which, being broke, mews a dull and po- 



* Thefe contain a phlogifton, which is difcovered by ex- 

 poling them to a quick and firong fire, in which they become 

 quite black interiorly, aiTuming the appearance of the com- 

 mon flints, not only in regard to colour, but alio in regaid 

 to hardnefs : but if heated by degrees, they are firft white, 

 and afterwards of a pearl colour. The fatter they feem to 

 be, which may be judged both by their feeling fmooth and 

 unctuous, and by their fhining, when fcraped with the nail, 

 they contain a larger quantity of the inflammable principle, 

 h is difficult to determine, whether this iirongly adherent 

 phlogiilon is the caufe of the above-mentioned pearl colour, 

 or prevents them from being burnt white in a firong fire : 

 yet no heterogeneous fubflance can be extracted from them, 

 except fand, which may be feparated from fome, by means, 

 of water, but which fand does not make out any of the 

 conilituent pans of the clavs . if they be boiled in aqua 

 regis, in order to extract any iron, they are found to lofe 

 their vifcoiity. In the lefs unctuous clays, I have found pure 

 quartz in greater and final ler grains; but Hill I would not 

 venture to affect a that one is produced from the other, accord- 

 ing to the rule I have laid down in Sect. ix. I. I have like- 

 wife found this fort, upon certain occalions, attract the phlo- 

 giilon in the fire. Thefe remarks may ferve as hints for the 

 lefs experienced, who have a mind to examine thofe clays, 

 which are of fo grsut conference for their ceconomical 



Vlfea. 



rous 



