PROFESSOR THOMAS GRAHAm's SCIENTIFIC WORK. 195 



with a saline solution. When the full hydration of the outer surface 

 extends through the thickness of the membrane, and reaches the inner 

 surface, it thex-e receives a check. The degree of hydration is lowered, 

 and tbe water must be given up by the inner layer of the membrane, 

 and it forms the osmose. * * * Far from promoting this separation of 

 water, the diffusion of the salt throughout the substance of the mem- 

 brane appears to impede osmose by equalizing the condition as to saline 

 matter of the membrane through its whole thickness. Th« advantage 

 which colloidal solutions have in inducing osmose, appears to depend in 

 l^art upon the low diffusibility of such solutions, and their want of power 

 to penetrate the colloidal septum." 



VI. 



Movements of Gases under pressure. Effusion and transjnration. — 

 The mechanical law of the passage of different gases nnder the same 

 pressure through a mere perforation, as of the passage of different liquids, 

 being that the velocities are inversely as the square roots of the specific 

 gravities, Mr. Graham subjected this law to an experimental verification, 

 and made known his results in a paper communicated to the Roj'al 

 Society in 184G. The mode of experimenting was as follows : A jar 

 standing on the plate of an air-pump was kept vacuous by continued 

 exhaustion, and a measured quantity of gas allowed to find its way into 

 the jar through a minute aperture in a thin metallic plate. The admis- 

 sion of 00 cubic inches of dry air into the vacuous, or nearly vacuous 

 jar, being arranged to take place in about 1,000 seconds, the times of 

 passage of the same volume of air were found not to vary from each 

 other by more than two or three seconds in successive experiments. 

 Operating with difierent gases, the relative times of passage, or of "effu- 

 sion," as it was denominated by Mr. Graham, proved to be approxima- 

 tively identical with the square roots of the specific gravities of the several 

 gases 5 or, in other words, their velocities of effusion were shown exper- 

 imentally to be inversely as the square roots of their specific gravities. 

 The rate of eiiusion of a mixed gas corresponded in most cases with the 

 calculated mean rate of its constituents ; but the rates of effusion of the 

 light gases, marsh gas and hydrogen, were very disproportionately re- 

 tarded by the admixture with them, even to a small extent, of the heavier 

 gases, oxygen and nitrogen. 



Passing from the study of the effusion of gases through a perforated 

 plate, Mr. Graham next submitted their " transpiration" through a 

 capillary tube to a similarly conducted experimental inquiry. His re- 

 sults were communicated to the Eoyal Society in two very elaborate 

 papers, " On the motion of gases," Parts I and II,* the first part con- 

 taining also his above-described results on the effusion of gases. With 

 a very short capillary, the relative rates of passage of different gases 

 were found to approximate to their relative rates of effusion ; but with 



* Philosophical Traasactious, 184G, p. 573 ; 1849, p. 349. 



