178 EFFECTS of HEAT [APPENDIX. 
a method different from that ufually employed in eftimating fpe- 
cific gravity. ; 
In the common method, the fubftance is firft weighed in air, 
and then in water ; the difference indicating the weight of wa- 
ter difplaced, and being confidered as that of a quantity of wa- 
ter equal in bulk to the folid body.. But as chalk, when {fatu- 
rated with water, is heavier, by about one-fourth, than when 
dry, it is evident, that its apparent weight, in water, muft be in- 
creafed, and the apparent lofs of weight diminifhed exactly to that 
amount. To have a juft eftimate, then, of the quantity of wa- 
ter difplaced by the folid body, the apparent lofs of weight muft 
be increafed, by adding the abforption to it. 
Two diftinét methods of taking {pecific gravity thus prefent 
themfelves, which it is of importance to keep feparate, as each 
of them is applicable to a particular clafs of fubjects. 
OnE of thefe methods, confifts in comparing a cubic inch of a 
fubftance in its dry ftate, allowing its pores to have their fhare 
in conftituting its bulk, with a cubic inch of water. 
Tue other depends upon comparing a cubic inch of the fo- 
lid matter of which the fubftance is compofed, independently of 
vacuities, and fuppofing the whole reduced to perfec folidity, 
with a cubic inch of water. 
Tuus, were an architect to compute the efficacy of a given 
bulk of earth, intended to load an abutment, which earth was 
dry, and fhould always remain fo, he would undoubtedly follow 
the firft of thefe modes: Whereas, were a farmer to compare 
the fpecific gravity of the fame earth with that of any other 
foil, in an agricultural point of view, he would ufe the fecond 
mode, which is involved in that laid down by Mr Davy. 
As our object is to compare the fpecific denfity of thefe re- 
fults, and to afcertain to what amount the particles have ap- 
proached 
