210 Effects of Heat modified ly Compression. 



Jn the common method, the substance is first weighed in 

 air, and then in water ; the difference indicating the weight 

 of water displaced, and being considered as that of a quan- 

 tity of water equal in bulk to the solid body. But as chalk, 

 when saturated with water, is heavier, by about one-fourth, 

 than when dry, it is evident that its apparent weight in 

 water must be increased, and the apparent loss of weight 

 diminished exactly to that amount. To have a just esti- 

 mate, then, of the quantity of water displaced by the solid 

 body, the apparent loss of weight must be increased by add- 

 ing the absorption to it. 



Two distinct methods of taking specific gravity thus 

 present themselves, which it is of importance to keep sepa- 

 rate, as each of them is applicable to a particular class of 

 subjects. 



One of these methods consists in comparing a cubic inch 

 of a substance in its dry state, allowing its pores to have 

 their share in constituting its bulk, with a cubic inch of 

 water. 



The other depends upon comparing a cubic inch of the 

 solid matter of which the substance is composed, indepen- 

 dently of vacuities, and supposing the whole reuueed to per- 

 fect solidity, 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 should always remain so, he would undoubt- 

 edly follow the first of these modes : whereas, were a farmer 

 to compare the specific gravity of the same earth with that 

 of any other soil, in an agricultural point of view, he would 

 use the second mode, which is involved in that laid down 

 by Mr. Davy. 



As our object is to compare the specific density of these 

 results, and to ascertain to what amount the particles have 

 approached each other, it seems quite evident that the first 

 mode is suited to our purpose. This will appear most di- 

 stinctly by inspection of the following table, which has been 

 constructed so as to include both. 



Tulle 



