390 ER DAVY ON THE SPECIFIC GRAVITY OF SUBSTANCES 



substance. When the exhaustion was nearly complete, the little mass of caoutchouc 

 was thrown into violent motion, resembling in its movements a piece of potassium 

 on the surface of water, being driven from side to side with strong effervescence or 

 ebullition. Even after many hours, bubbles continued to be disengaged : on their 

 cessation, I found its specific gravity .93. But as, on examining this caoutchouc 

 with the microscope, it also was found to contain minute cavities, which might be 

 filled with the vapour of ether or of alcohol, if not vacua, the specific gravity, as 

 ascertained, was liable to the same objection as in the case first referred to of 

 common caoutchouc. 



To endeavour to obviate the interfering circumstances in this experiment, 

 another was made. To an etherial solution not saturated, alcohol in considerable 

 quantity was added, in a manner to prevent the particles of caoutchouc, as they 

 were precipitated, from cohering and forming a mass. After the addition of a 

 portion of water, the mixture was subjected to the air-pump, and was kept under 

 the exhausted receiver till the greater part of the spirit had evaporated. The 

 fluid was turbid from particles of caoutchouc suspended in it. Transferred to a 

 tube and carefully watched, some of them, chiefly the largest, were seen to ascend ; 

 others were seen to descend. The specific gravity of the fluid was found to be 

 97. This result seems to be in favour of the conclusion that the true specific 

 gravity of caoutchouc differs but little from that of water, as in all probability 

 even the smallest visible particles contained cavities in which might be included 

 the lighter substance either of ether or of alcohol. 



The specific gravity of camphor, according to BRISSON, is -9887. Subjected 

 to the air-pump, floating on distilled water, air is disengaged from it, and the 

 mass of camphor gradually sinks in the water ; but unless very small, not entirely. 

 On close inspection it may be seen commonly to be buoyed up by minute globules 

 of air, either adhering to it or included in its substance. Now, if the mass be 

 broken in the water and reduced to a coarse powder, and again submitted to the 

 pump, after the exhaustion, many of the little fragments will be seen to have sub- 

 sided, and some will be seen at the bottom. If, however, the warm hand be ap- 

 plied to the bottom of the vessel, then the bits of camphor will be found to rise 

 and fall with the ascending and descending currents produced in the fluid. The 

 inference from this experiment obviously is, that the specific gravity of camphor 

 exceeds that of water, but only in a very slight degree. Adding salt, I find the 

 particles free from air ascend and descend with the currents, excited by the par- 

 tial application of heat, when the water has acquired the specific gravity 1'005, 

 at about the temperature 50 ; which therefore may be concluded to be about that 

 of the substance itself. And confirmation of this was obtained by dissolving a 

 portion of camphor in alcohol, precipitating by water deprived as much as pos- 

 sible of air, and adding a portion containing particles of the precipitated camphor 

 to the salt and water. Many of them remained stationary below the surface, 



