SPECIFIC GRAVITY OF POROUS SUBSTANCES, ] Q3 



described and analysed, a specific gravity of 2-322; while specific gravity 



Mr. Brisson gives no more than 1*232 for that of the piece of porous sub " 



„.. . ° . . , T . . . ' r stances, 



ot tittering stone, which 1 have mentioned as serving to es- 



tabish a similarity. But it must be considered, that, to ob- from supposed 

 tain this result, Mr. Brisson adds to the weight necessary to corrections, 

 restore the equilibrium, when the substance is immersed in 

 water, the freight of the quantity of water that has penetrated 

 it. Such was the method adopted by the author, for sub- 

 stances capable of imbibing water, which appears to me to 

 require a farther examination, though followed by many na- 

 tural philosophers. It is true Mr. Brisson gives to the spe- 

 cific gravities of these same substances a second expression, 

 derived from a calculation, in which the absolute weight, or 

 the weight taken in air, is increased by the weight of the 

 water absorbed. But neither of these expressions can <rive 

 the true ratio of the mass of matter to the actual place it 

 occupies; since in the first, that water is reckoned as dis- 

 placed, which only succeeds to the air that before occupied 

 the pores, and which ascends in bubbles from every part of 

 the surface; and in the second, the weight of the mass is 

 confounded with that of the fluid employed to circumscribe 

 its solid parts. 



In fact, if I had proceeded on this principle, I should But no correc- 

 have found the specific gravity of the filtering stone no more 5JUSrt£ 

 than 1*813, which comes very near to that assigned by Mr. stance is sum- 

 Brisson. On the other hand, if we apply to the data of his jjjjjjj ^ 

 experiment the simple calculation of dividing the weight of 

 the body in air by the weight necessary to add to restore 

 the equilibrium when it was weighed in water, we shall have 

 as the quotient 2*391, consequently still a little more than 

 the same calculation for the stone I examined gave me. - 



Sand stones and filtering stones are not the only fossil Other fossils 

 substances, that receive into their pores the surrounding penetrable by 

 medium. Chalcedonies, pitchstones, steatites, asbestos, water " 

 mesotype, schists*, some micas, and even, according to 

 Gerhard, some varieties of jade, are more or less penetrable 

 by water. 



* Mr. Ludicke has described some hard schists, which he found to 

 imbibe and part with moisture so regularly, that they were capable of 

 answering the purposes of an hygrometer. 



This 



