388 DR DAVY ON THE SPECIFIC GRAVITY OF SUBSTANCES 



thoroughly dried. It floated on water, in the exhausted receiver, about thirty 

 hours ; and continued to give off air extremely little indeed in quantity until 

 about the thirtieth day, reckoning from the commencement of the exhaustion.* 



There is a small number of other substances generally believed to be lighter 

 than water, respecting which doubt may be entertained, such as cork, caoutchouc, 

 camphor, wax, spermaceti, cholesterine, stearine, which, like the preceding, may 

 owe their apparent lightness to entangled air. 



To endeavour to determine this question, I have made some experiments, the 

 results of which I shall noAV have the honour of submitting to the Society, be- 

 lieving the subject to be deserving of some attention, practically considered, espe- 

 cially in connexion with the examination and analysis of certain vegetable and 

 animal compounds, the oily and fatty contents of which are daily becoming more 

 interesting, in connexion with theoretical views respecting elementary cells and 

 their nuclei. 



The great buoyancy and apparent extreme lightness of cork is well known : 

 its specific gravity is stated to be as low as .2400.1 When subjected to the air- 

 pump, much air is disengaged from it ; it subsides a certain way in the water, 

 but does not sink. A portion of cork weighing 12.4 grains, was kept under the 

 exhausted receiver from the 23d of January until the 3d of April, when it conti- 

 nued to float ; and even minute portions of this substance, not exceeding one 

 tenth of a grain in weight, appear incapable of being sunk by the action of the 

 air-pump.:]: This, it may be conjectured, is owing to the elastic cellular structure 



* Reduced to powder, after having been subjected to the air-pump, and weighed hydrostatically, it 

 was found to be of the specific gravity 2.41, which is about that of obsidian, the mineral substance 

 from which, it would appear, that pumice is formed by the action of volcanic fire. As no air was disen- 

 gaged when the pumice was crushed under water, it seems probable, from the circumstance of its specific 

 gravity being increased by its cells having been broken, that some of them may be destitute even of 

 air. This brings to my recollection the result of an experiment made many years ago, on exposing 

 obsidian to a high temperature in a gun-barrel, in which I assisted a distinguished member of this 

 Society, Sir GEORGE MACKENZIE. The air disengaged from the obsidian had a distinct smell of nitrous 

 acid gas. Now, supposing that this acid is always set free in the production of pumice from obsidian, 

 part of it may be re-absorbed, and tend perhaps, with steam, to form the minute vacua which I have 

 supposed may exist in pumice, vacua, the existence of which it is easy to imagine, considering the na- 

 ture of the substance, in reality a vesicular glass, and differing chiefly from obsidian, or, as it has been 

 significantly called, volcanic glass, in its vesicular condition. This is well displayed by the microscope, 

 under which, with a high power, its minute fragments appear as transparent glass, in some of which 

 cavities are distinguishable. 



f Henry's Chemistry, vol. ii., p. 506. 



J Whether cork kept in water unaided by pressure, would ever sink, seems very doubtful ; probably 

 it would continue to float so long as the plates constituting its cells retained their integrity and elasti- 

 city, that is, so long as its substance resisted decomposition. The portion of cork, the subject of the 

 experiment described in the text, which weighed in air 12.4 grains, after having been in water, under 





