1826.] 



Scientific Notices — Miscellaneous. 



313 



Miscellaneous. 



9. Description of a new Instrument to ascertain the Specific 



Gravity of Fowders. 



Prof. Leslie lately exhibited a very interesting experiment in 

 his Class Room, with a new instrument to ascertain the specific 

 gravity of powders, and all kinds of solid substances which will 

 not bear immersion in water. 



The instrument consists of a glass tube a e, 

 about three feet long, and open at both ends. 

 The wide part a 6 is about four-tenths of an 

 inch in diameter, the part b e about two- 

 tenths. The two parts communicate at b by 

 an extremely fine slit, which suffers air to 

 pass, but retains sand or powder. The mouth 

 at a is ground smooth, and can be shut so as 

 to be air tight by a small glass plate (shown 

 a little above it in the figure). The substance x 



whose specific gravity we wish to find — 

 suppose it to be sand — is put into the wide 

 part of the tube, or the cylindrical cavity a b, 

 which may either be filled to the top or not. 

 The tube being then held in a vertical posi- 

 tion, the narrow part is immersed in an open tube or vessel x, 

 filled with mercury, till the mercury rises both inside and outside 

 to the gorge at b. The lid is then fitted on air-tight at a. In 

 this state it is evident that there is no air in the tube except 

 what is mixed with the sand in the cavity a b. Now suppose 

 the barometer at the time to stand at 30 mches, and that the 

 tube is lifted perpendicularly upwards till the mercury stands in 

 the inside of 6 e at a ponit c, 16 inches* above its surface in the 

 open vessel x-. It is evident then that the air in the inside of 

 the tube is subjected to a pressure of exactly half an atmo- 

 sphere, and of course it dilates and fills precisely twice the space 

 it originally occupied. It follows too, that since the air is dilated 

 to twice its bulk, the cavity a b contains just half of what it did 

 at first, and the cavity b c now containing the other half, the 

 quantity of air in each of these parts of the tube is equal. In 

 other words, the quantity of air in be is exactly equal to what is 

 mixed with the sand in a b, and occupies precisely the same 

 space which the whole occupied before its dilatation. Let us 

 now suppose the sand to be taken out, and the same experiment 

 repeated, but with this difference, that the cavity a b is filled 



• 'Vt'henthe barometric column is 29 inches, the height is I4j^, and so on. This is 

 regulated by a scale. 



