• i-jS On the Ab/orienl Powers of difertnt Earths. 



their difpofition to imbibe moifture. Manures perhaps a£\: as ftimulants merely, the car- 

 bonaceous matter of plants is derived from the atmofphere, and the earth affords expanfion 

 for the fibrils of the roots, and fupplies them with the aqueous element. Rotten whin- 

 ftone conftitutes the body of the foil in this fertile county, and it manifefts among the 

 ftony compounds a fuperior degree of the power of abforption. 



Torrefaftion feems remarkably to diminiflti the faculty of the earthy fubftances to attraft 

 moifture. Clay, roafted in a ftrong fire, from 68 gives only 35 degrees by the hygrome- 

 ter, and after being urged in a blackfmith's forge, affords no more than 8 ; whinftone, 

 which in ordinary cafes has a power equal 1078°, (hews only 23°, after cxpofure to the 

 fierce heat of a forge. Nor is the efFeft occafioned by any partial or incipient vitrification, 

 for fandftone, which has undergone a violent ignition, fhews likewife a fimilar change of 

 property. If bodies therefore fufFered no alteration of contexture from the imprefhons of 

 the atnaofphere, geologifts would be furniflied with one certain criterion, to decide whe- 

 ther a fofhl owed its formation to the agency of fire or water. It is almoll fuperfluous to 

 obferve, how vague and inconclufive are the arguments ufually employed by the contending 

 feels of Neptutiifts and Plutonifts. 



But although the mode of experimenting above defcribed fufRciently marks their difcri- 

 minating qualities, it does not exhibit the full efFecfl of the deficcated powders ; fince, the 

 hygrometer and the earth being included together, while the latter abftrafts moiflure from 

 the air within the receiver, the former by its wetted ball continually imparts humidity, 

 which notwithftanding the extenfive furface of abforption, muft always enfeeble the per- 

 formance in proportion to the time of a£tion. The moft accurate way of experimenting, 

 is to throw the powders, either from the phials, or direftly on its being withdrawn from 

 the fire, into a large flattened glafs vcfTel with a narrow neck, which is corked up for feveral 

 hours, and the condition of the included air may then be examined by letting down a fmall 

 hygrometer fufpended by a thread from the flopper. For want of more fuitable apparatus, 

 I ufed wine decanters, and having provided myfelf with fpecimens of the primitive earths^ 

 I began with determining their relative abforbent powers. The refuUs at the fame tem- 

 perature, of 16° centigrade, were as follow : 



Carbonate of ftrontian .... 23° 

 Carbonate of barytes .... 32 



Quartz * 40 



Marble ...... 70 



Carbonate of niagnefia ; ... 75 

 Alumine ...... 84 



This furniflies another proof that ftrontlanite is an independant earth, fince it ftands 

 apart from the refl. Quartz in fmall rounded fragments, like pigeon's eggs, picked up on 

 the beach, gave almoft the fame refult as when reduced to a powder. In general, the only 

 ufc of pounding the ftones is to accelerate their performance. It might be prefumed, that 

 the mixtures of thofe earths would produce intermediate efFefis ; yet equal parts of Clex 



and 



