20G PROFESSOR THOMAS GRAHAM's SCIENTIFIC WORK. 



"Lastly, molecular or diflfusive mobility has an obvious bearing upon 

 the communication of heat to gases by contact with liquid or solid sur- 

 faces. The impact of the gaseous molecule upon a surface possessing a 

 different temperature appears to be the condition for the transference of 

 heat, or the heat movement, from one to the other. The more rapid the 

 molecular movement of the gas, the more frequent the contact with con- 

 sequent communication of heat. Hence, probably, the great cooling 

 power of hydrogen gas as compared with air or oxygen. The gases 

 named have the same specific heat for equal volumes, but a hot object 

 placed in hydrogen is really touched 3.8 times more frequently than it 

 would be if placed in air, and 4 times more frequently than it would be 

 if placed in an atmosphere of oxygen gas. Dalton had already ascribed 

 this peculiarity of hydrogen to the high 'mobility' of that gas. The 

 same molecular property of hydrogen recommends the application of 

 that gas in the air-engine, where the object is to alternately heat and 

 cool a confined volume of gas with rapidity." 



VIII. 



Passage of gases through colloid septa. — In 1830, Dr. Mitchell, of Phila- 

 delphia, discovered a power in gases to penetrate thin sheet India 

 rubber ; and, noticing the comiiaratively rapid transmission of carbonic 

 gas through the rubber, associated this observation with the further one 

 that a solid piece of India rubber is callable of absorbing its own volume 

 of carbonic gas, when left in contact with excess of the gas for a suffl- 

 cieut leugth of time. By means of a suitable arrangement, Dr. Mitchell 

 found that various gases passed si)ontaneousl3^ through a caoutchouc 

 membrane into an atmosphere of ordinary air with different degrees of 

 velocity — that as much of ammonia gas was transmitted in 1 minute as 

 of carbonic gas in oi minutes, as of hydrogen in 37^ minutes, and as of 

 oxygen in 113 minutes. Soon after their publication, these results were 

 ably commented on and extended by Dr. Draper, of New York ; and, 

 altogether, they attracted considerable attention in scientific circles. 

 One of Mr. Graham's earliest observations — having reference to the 

 spontaneous passage of carbonic gas into a moist bladder of air, so as 

 ultimately to burst the bladder — had obviously a very close connection 

 with Dr. ]\Iitcliell's results, and received from Mr. Graham in 1829 the 

 same explanatioii that in 18GG he gave to his own India rubber experi- 

 ments, the account of which he communicated to the Royal Society in a 

 paper "On the absorption and dialytic separation of gases by colloid 

 septa." * In his experiments on the j)enetration of different gases, 

 through septa of India rubber, into a vacuum, Mr. Graham employed 

 a tube considerably^ exceeding in length the barometric column, open at 

 one end and closed at the other by a thin film of caoutchouc stretched 

 over a plate of highly porous stucco. On filling this tube with mercury, 



* Philosophical Transactions, 18i6, p. 399. 



