152 



EXPEEIMENTS OF DALTON AND GRAHAM. 





Graham's ex 





end of which dips beneath some water. A torrent of gas bubbles will 

 escape. 



Mr. Dalton demonstrated that if a light gas be placed above a heavy 

 Daiton'sexper- gas in a suitable apparatus, the former, notwithstanding its 

 ^~ nt . on t ? e levity will descend, and the latter, notwithstanding * its 



diiiusion 01 f~ 



gases. weight, will rise, and a complete and uniform intermixture 



will result. By such experiments he was led to believe that 

 gases act as vacua to one another, and correctly explained the 

 uniform composition of the atmosphere on this property of dif- 

 fusion, or tendency of its constituents to intermix. 



Thus, if a vial filled with hydrogen be placed with its 

 mouth downward over the mouth of a vial of the same size 

 containing carbonic acid gas, as shown at A, <?, Fig. 67, in the 

 course of a few moments the diffusion will be complete, and 

 if the mixture in either vial be examined, it will be found to 

 contain equal quantities of the gases. 



Professor Graham extended Dr. Priestley's observations on 

 the passage through porous barriers. The sub- 

 stance he chiefly employed was a mass of dry 

 plaster of Paris. This enabled him to prove that Di ini5ion of 

 in the case of different gases diffusion takes place at different 

 rates, which are dependent on the density of the gas. Per- 

 haps the most satisfactory method of illustrating this class of 

 Diffusion results is by taking a porous earthenware cup, 

 throughporous a a; Fig. 68, such as is used in Grove's voltaic 

 battery, drying it perfectly, and cementing into 

 its mouth an open glass tube, b, three quarters of an inch 

 in diameter, and a foot or more long. A wide-mouthed bot- 

 tle, c c, being placed as a temporary cover over the porous 

 cup, it may be filled with hydrogen gas by displacement ; 

 and if the end of the glass tube be put into water contained 

 in a reservoir, d, the water will rush up the moment the 

 bottle is removed. When this motion is completed, if a jar 

 of hydrogen be held over the porous cup, the water will be 

 driven down with great rapidity, and a number of air-bub- 

 bles quickly escape. The extraordinary speed with which 

 a gas will flow in and out of pores could not be better dis- 

 played. This rapidity of motion is an element with which 

 the physiologist has to deal, as we shall presently find. 



Even when the texture of the substance is much closer, and the pores 

 Diffusion of extreme minuteness, similar results can be obtained, as was 

 through in- shown in the experiments of Dr. Mitchell, of Philadelphia, who 

 employed thin sheets of India-rubber. If, over the mouth of 



Fig. 68. 



through 

 earthenware. 





