itz EFFECTS of HEAT [Appendix. 



The fpecific gravities afcertained by the new mode, and ex- 

 prefTed in Column X. correfpond very well to the idea which is 

 formed of their comparative denfities, from other circumftances, 

 their hardnefs, compact appearance, fufceptibility of polifli, and 

 weight in the hand. 



The cafe is widely different, when we attend to the refults of 

 the common method contained in Column IX. Here the fpe- 

 cific gravity of chalk is rated at 2.498, which exceeds confider- 

 ably that of a majority of the refults tried. Thus, it would ap- 

 pear, by this method, that chalk has become lighter by the ex- 

 periment, in defiance of our fenfes, which evince an increafe of 

 denfity. 



This fingular refult arifes, I conceive, from this, that, in our 

 fpecimens, the faculty of abforption "has been much more decrea- 

 fed than the porofity. Thus, if a piece of crude chalk, whofe 

 fpecific gravity had previoufly been afcertained by the common 

 mode, and then well dried in a heat of 212°, were dipped in var- 

 nifh, which would penetrate a little way into its furface ; and, 

 the varnifh having hardened, the chalk were weighed in water, 

 it is evident, that the apparent lofs of weight would now be 

 greater by 23.61 per cent, of the dry weight, than it had been 

 when the unvarnifhed chalk was weighed in water ; becaufe the 

 varnifh, clofing the fuperficial pores, would quite prevent the 

 abforption, while it added but little to the weight of the mafs, 

 and made no change on the bulk. In computing, then, the fpe- 

 cific gravity, by means of this lafl refult, the chalk would appear 

 very much lighter than at firfl, though its denfity had, in fact, 

 been increafed by means of the varnifh. 



A similar effect feems to have been produced in fome of 

 thefe refults, by the agglutination or partial fufion of part of 

 the fub fiance, by which fome of the pores have been fhut out 

 from the water. 



This 



