404 MB. T. GRAHAM ON THE ABSORPTION AND 



747 millims., therm. 21°-1. The ultimate replacing volumes are here as 1 to 4'7. In 

 gas-diffusion they are as 1 to 3"8. 



A balloon filled with air subsided in forty-eight hours from 150 to 147 millims. in 

 diameter, from the mechanical eifect alone of the elasticity of the membrane in com- 

 pressing the enclosed gas. These little balloons vary from 0-75 to 1 grm. in weight. 

 Supposing the form to be truly spherical, a balloon of 150 millims. in diameter would 

 have a surface of 0"0706 square metre (5-905 inches in diameter and 0*08454 square 

 yard of surface). Supposing the balloon to be 1 grm. in weight, the thickness of the 

 membrane will be ^p-ese of a millim., with a specific gravity =1, or 7,5^01 of a miUim., 

 with a specific gravity =0-93, the admitted density of pure rubber. This last is a thick- 

 ness of ^93o.Q of an inch, or it would require nearly 2000 such films, laid upon each 

 other, to form the thickness of a single inch. Yet such a film of rubber appears to have 

 no porosity, and to resemble a film of liquid in its relation to gases — differing entirely in 

 this respect from a thin sheet of paper, graphite, earthenware, or even gutta percha, as 

 will appear hereafter. These last enumerated bodies appear all to be pervaded by open 

 channels or pores, sufficiently wide to allow gases to be projected through by their own 

 proper molecular movement of diff'usion. But liquids and colloids have an unbroken 

 texture, and afford no opportunity for gaseous diffusion. They form even in the thinnest 

 film an impervious barrier to gas. 



The penetration of rubber is much affected by temperature, and apparently in two 

 different ways at the same time. An increase of temperature no doubt renders all gases 

 less easily liquefied by pressure, and consequently less considerably absorbed by any liquid 

 or colloid. But such an influence of heat appears to be counteracted in rubber by the 

 tendency of that colloid to become more soft when heated, and to acquire more of 

 liquid and less of solid properties. Certainly the rubber film becomes more and more 

 permeable to gases as the temperature is elevated, within a moderate range. This was 

 distinctly observed in operating with silk cloth varnished on one side with rubber, such 

 as is sold as a waterproof material. Without anticipating a detail of the experiments, 

 it may be stated in general terms that the same specimen of rubber was penetrated by 

 air from the atmosphere passing into a vacuum, at the following rates per square metre 

 of surface : — 



At 4° C, by 0-56 cub. centim. of air in 1 minute. 



At 14° C, by 2-25 



At 60° C, by 6-63 



The volumes of gas are all reduced to barom. 760 millims. and therm. 20° C. 



Such numbers are probably not strictly constant ; for it appears that the effect of tem- 

 perature upon rubber is much influenced by the length of time that the temperature is 

 continued, the change in degree of softness with change of temperature requiring 

 hours, or even days, fully to complete it. The rigidity of rubber under cold and its 

 softening under warmth are well known to take place in a slow and gradual manner. 



With the softening of rubber by heat, the retentive power of that substance for gases 



