CONSIDERED LIGHTEE THAN WATER. 38.Q 



of cork offering resistance to the escape of air, when highly rarified, similar to 

 that presented by a strong solution of gum-arabic, or any other viscid fluid acted 

 on by the air-pump, when we see, after the pump has been worked some time, 

 and the exhaustion is as complete as it can well be made, that bubbles rise, on 

 which the farther working of the pump seems to have little effect, and which ap- 

 pear to break rather in consequence of evaporation than of exhaustion. In accord- 

 ance with this view, when the cells of the cork are forcibly compressed and bro- 

 ken down, then their substance ceases to be buoyant, and the cork sinks readily in 

 water. The effect is easily shewn by compressing and breaking up small portions, 

 as by forcibly crushing them in a mortar under water. I find, after this has been 

 done, that cork sinks not only in water, but also in a saturated solution of common 

 salt, which is of the specific gravity 1.148, and in sulphuric acid of specific gra- 

 vity 1.5, at which strength the acid has no immediate charring effect. This last 

 result would seem to indicate that its specific gravity exceeds 1.5. If the base of 

 cork be considered as wood (a conclusion I am disposed to adopt, rather than the 

 idea that it is a distinct substance, suberine), it is probable that its specific- 

 gravity is as high even as 1.6, which I believe to be that of the matter wood or 

 lignin in its purest form, being, as I find, that of cotton and of linen.* 



The specific gravity of caoutchouc, according to BRISSON, is .93. I have 

 found it, when its outer black pellicle has been removed, even lower, viz. .91. 

 Under the microscope, using a high power, this specimen appeared to have within 

 its substance minute cavities, which, from the transparency of the mass, were 

 sufficiently distinct. From the properties of caoutchouc, it could not be expected 

 that any air contained in it, in closed cavities, could be extracted, either entirely 

 or even in considerable part, by the air-pump, or by boiling, or by compression. 

 Solution in ether, and precipitation by alcohol and the addition of water purged 

 of air, seemed to afford a probable means of determining the question, whether 

 or not the presence of ah* in this substance is connected with its lightness. 



To a hot saturated solution of caoutchouc in sulphuric ether, alcohol was 

 added ; the caoutchouc thrown down, resembling in appearance a mass of fibrin 

 separated from the blood by stirring, was taken up by a forceps and transferred 

 to water that had ceased to give out air under the exhausted receiver, and was 

 immediately acted on by the air-pump. The effect of the removal of the atmo- 

 spheric pressure on it was remarkable, owing, no doubt, to the ether adhering to its 



the exhausted receiver 33 days, had increased in weight, from the absorption of the water, to 20.5 grains ; 

 and after 22 days more, its farther increase was only 1 grain. 



* I have found the specific gravity of cambric carefully freed from air 1.600 ; of hemp cord, 1.560 ; 

 of fine cotton cloth, 1.605; and of cotton thread, 1.61, at 50 F. The cambric and cotton thread were 

 first boiled in distilled water, and then subjected to the air-pump before weighing in water ; they were 

 thoroughly dried before being weighed in air, and weighed whilst still warm. The cord and thread were 

 treated in the same manner, excepting that they were not previously boiled. 



