425 



The specific gravity of oak-wood, after having been kept under the 

 exhausted receiver till it sunk in water, and ceased to give off air, he 

 found to be (inclusive of hygronietric moisture) 1.58 ; that of deal, 

 1.18, or when crushed about 1.5 ; that of the pith of the elder, 1.45 ; 

 and that of pumice, 1.94, or crushed, 2.41, which is nearly the same 

 as that of obsidian, from which pumice appears to be formed by the 

 action of volcanic fire. 



Reasoning analogically, the question naturally occurred, are there 

 not other substances, commonly considered lighter than water, which 

 are so only in appearance, owing, it may be, to air adhering to their 

 surface, or included in them, such as cork, caoutchouc, camphor, wax, 

 spermaceti, cholesterino, stearine ? 



Cork, he finds, continues to float on water, after having been kept 

 under the exhausted receiver nearly two months. This he attributes 

 to the elasticity of the plates constituting its cells, confining a very 

 minute quantity of air, that cannot be extracted by the pump. 

 When the cells are broken, it sinks in water. From indirect ex- 

 periments, ho infers its specific gravity to be about 1.6, or nearly 

 the same as that of lignin. 



In common caoutchouc, ho detected the presence of air by the 

 microscope, which, owing to the peculiar quality of this substance, 

 could not be separated either by the air-pump, or by compression, 

 or by boiling. Dissolved in ether, and precipitated by alcohol, he 

 found it apparently of specific gravity .97 ; allowing for circumstances 

 interfering with accuracy of result, he conjectures that the specific 

 gravity of caoutchouc is about the same as that of water. 



The specific gravity of camphor, he concludes from his experi- 

 ments, to exceed a very little that of water, viz. as 1.005 to 1.000; 

 that of bees' wax, and also that of spermaceti and of stearine, to be 

 about the same as that of water at the temperature 50°, though 

 considerably lighter at the boiling temperature of water ; and that 

 of cholesterino to be a little higher, viz. 1.0102. 



In conclusion, he expresses the opinion, that attention to the spe- 

 cific gravities of the substances under consideration, may be useful 

 in conducting chemical analyses of compound bodies in which they 

 may be contained ; that a consideration of the cause of their appa- 

 rent lightness, viz. included or adhering air, may help to explain 

 some rather obscure phenomena, — as the raising of cream, i:c. ; 

 and that attention may be deserving of being paid to the same cause, 

 as exemj)lificd in the substances treated of, whenever, in works of 

 art, unusual lightness or buoyancy is a desideratum. 



