314 Scientific Notices — Miscellaneous. [April, 



with air only. It is obvious that as the quantity of air is greater 

 than it was when part of the cavity was filled with sand, it will, 

 when dilated to double the bulk under a pressure of 16 inches, 

 occupy a larger space than in the other experiment, and the 

 mercury will only rise, let us suppose to d. But let it be remem- 

 bered that the attenuated air in the narrow tube always occu- 

 pies exactly the space which the whole occupied under the ordi- 

 nary atmospheric pressure. Now^ this space is in the one case 

 the cavity b c, and in the other b d. Hence it clearly follows, 

 that the cavity c d, which is the difference between these, 

 is equal to the bulk of the solid matter in the sand. Now by mark- 

 ing the number of grains of water held by the narrow tube b e 

 on a graduated scale attached to it, we can find at once what is 

 the weight of a quantity of water equal in bulk to the solid 

 matter in the sand, and by comparing this with the weight of 

 the sand in air, we have its true specific gravity. 



Such is the Professor's process, which appears to us remark- 

 ably ingenious, as well as beautifully simple, and we shall see 

 from some of the results which it has already afforded, that it 

 must furnish important aid to the natural philosopher in his 

 researches. On one point a few words of explanation may be 

 required. The Professor is aware that some solid bodies, such 

 as charcoal, hold much condensed air in their pores, and since 

 the probability is, that when reduced to powder they still retain 

 this property in some degree, he obviates the chances of error 

 arising from this source, by comparing the dilatation which takes 

 place under different degrees of pressure — under 10 inches, for 

 instance, and 20, or under 7J- and 15. 



Charcoal is known to be precisely the same in its chemical 

 nature with the diamond ; but its porosity renders it so hght, 

 that the specific gravity assigned to it in books is generally 

 under 0*5, that is, less than half the weight of water, or one- 

 seventh part of the vveight of diamond. Prof. Leslie has, how- 

 ever, taken its specific gravity in the state of powder by the 

 instrument we have described, and he finds that its weight 

 actually exceeds that of diamond — that it is one-half greater 

 than that of whinstone — and of course that the substance is 

 more than seven times heavier than it has been hitherto supposed. 



The specific gravity of mahogany is generally put down as 

 1*06. Prof. Leslie finds that of mahogany saw-dust to be 1'68, 

 or two-thirds more. He found that of wheat flour to be 1*56, of 

 pounded sugar 1*83, of common salt 2*15. The last of these 

 agrees very accurately with the common estimate. Writing 

 paper rolled hard up with the hand was found to have a specific 

 gravity of 1-78, and to occupy less than one-third of the space 

 it apparently filled. One of the most remarkable results found, 

 is that relating to volcanic ashes, a substance which seems very 



