182 | EFFECTS of HEAT [APPENDIXx. 
- Tue fpecific gravities afcertained by the new mode, and ex- 
prefled 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 polifh, and 
weight in the hand. 
Tue 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. 
Tuis fingular refult arifes, I conceive, from this, that, in our 
{pecimens, 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 laft refult, the chalk would appear 
very much lighter than at firft, though its denfity had, in fact, 
been increafed by means of the varnith. 
A simILAR effeét feems to have been produced in fome of 
thefe refults, by the agglutination or partial fufion of part of 
the fubftance, by which fome of the pores have been fhut out 
from the water. 
Tuis 
