]04 SPECIFIC GRAVITY OF FOROUS SUBSTANCES'. 



Spec gravity This property unquestionably ought to be noticed in de- 

 sought by the sc^jng them: it makes a part of the characters, the aid of 

 naturalist, as ;■ © r 



illustrating the which is requisite to the naturalist in discriminating species; 

 nature of bo- b ut w ] ien } ie see k s the true specific gravity of any substance, 

 it is to acquire a more intimate knowledge of its nature, not 

 to derive from the measure of a surface full of pores, and 

 roughened with asperities, a gross calculation of the solidity 

 of its mass, as if his object were the estimation of a load. 



The problem, which it is of real importance to the pro- 

 gress of sciense to solve, is to determine the exact ratio, 

 that the proper substance of the body under examination 

 bears to the bulk of its contiguous parts, that leave no more 

 spaces into which the surrounding fluid can have access. 

 The water, which is absorbed as the air escapes, can no more 

 be considered as water displaced by the solid, than that im- 

 bibed by a sponge; and we should fall into a great error, if 

 Spec °rav of we wer e to estimate its density on this principle. It would 

 soluble sub be superfluous to say, that in all cases we suppose the water 

 stances, found tQ j iave nQ c ] iem i ca i ac ti n, as it has on salts; for then the 

 by Say s stere- 

 ometer; hydrostatic balance could not even give an approximation to 



the truth, and we must have recourse to Say's stereometer* ; 



or if we have not this ingenious instrument, which is not yet 



or by weighing * n vei T general us e, we must employ a fluid that has no ac- 



ina fluid, that tion on the subject to be examined, as for instance, water 



Slve^o^is'al- completely saturated with the same salt. Thus I used a 



ready saturat- saturated solution of nitrate of potash, when I was engaged 



s CTunpomiei- 1 " a ^ ie ^ ear **' as member of the committee appointed by 



in solution of the minister at war, to give a comparative table of the spe- 



nUre - cine gravities of all the different kinds of gunpowder used 



in the fleets or armies of various nations. 



The same principles led me to suspect, a few years ago, 

 Eirour in re- * ne eiTOUr '> i" ato which most mineralogists had fallen, in 

 gard to pumice ascribing to pumice stone a specific gravity even inferior to 

 that of water. Mr. Klaproth observes, in his analysis of 

 that of Lipari, that, though it contains more than 0-17 of 

 alumine, it is not at all attacked by acids : this, added to the 

 hardness we find in its smallest particles, though they are 

 easily separable, indicate a state of combination inconsistent 



* See a description of it, Annales da Chimie, Vol. XXill. p. 5. 



with 



