178 EFFECTS of HEAT [Appendix. 



a method different from that ufually employed in estimating 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 eflimate, then, of the quantity of wa- 

 ter difplaced by the folid body, the apparent lofs of weight mufl 

 be increafed, by adding the abforption to it. 



Two diftinct methods of taking fpecific gravity thus prefent 

 themfelves, -which it is of importance to keep feparate, as each 

 of them is applicable to a particular clafs of fubjecls. 



On e 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. 



The 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 perfect folidity, 

 with a cubic inch of water. 



Thus, 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 



