512 



NATURE 



{Oct. 5, 1876 



to the height of about 2% Inches above the outer level. Tn 

 repeating Dobereiner's experiments and varying the circum- 

 stances, Mr. Graham discovered that hydrogen never escaped 

 outwards by the fissure v/ithout a certain portion of air pene- 

 trating inwards, but with this essential difference, for every 

 volume of air which penetrated into the vessel 3'8 volumes of 

 hydrogen escaped. 



The apparatus consisted of a graduated glass tube nearly an 

 inch in diameter, having one end closed by a porous diaphragm 

 of plaster of Paris. This tube was filled with the gas to be 

 examined, and the rise of the mercury indicated the rate at 

 which the interchange of gas and external air took place. He 

 also interposed a bulb two or three inches in diameter between 

 the diaphragm and the graduated tube vsdth a view of increasing 

 the capacity of the instrument, and of avoiding the interference 

 of vapour. In this paper he traced the relation which diffusion 

 bears to the mechanism of respiration, but time will not permit 

 me to consider this question. 



These early results were repeated and greatly extended in a 

 paper " On the Molecular Mobility of Gases," ^ but in the ex- 

 periments there described, thin plates of compressed graphite 

 were principally used. The paper is chiefly remarkable for the 

 clear enunciation of the fact that diffusion is a molecular, and 

 not a ;«ajj movement, for Mr. Graham observes : " The pores 

 of artificial graphite appear to be so minute that gas in mass 

 cannot penetrate the plate at all. It seems that molecules only 

 can pass, and they may be supposed to pass wholly unimpeded 

 by friction, for the smallest pores that can be imagined to exist 

 in graphite must be tunnels in magnitude to the ultimate atoms 

 of a gaseous body. The sole motive agency appears to be that 

 intestine movement of molecules which is now generally recog- 

 nised as an essential property of the gaseous condition of matter. 



"According to the physical hypothesis now generally received, 

 a gas is represented as consisting of soUd and perfectly elastic 

 spherical particles or atoms, which move in all directions and 



Fig. 9. 



are animated with different degrees of velocity in different 

 gases." ... If the vessel containing the gas "be porous like a 

 diffusiometer, then gas is projected through the open channels 

 by the atomic motion described, and escapes. Simultaneously, 

 the external air is carried inward in the same manner, and takes 

 the place of the gas which leaves the vessel. To this atomic or 

 molecular movement is due the elastic force, with the power to 

 resist compression possessed by gases." 



In order to demonstrate the diffusion of gases it is necessary 

 to exaggerate the conditions of Mr. Graham's experiments. In- 

 stead of employing a tube closed with a disc of plaster of Paris, 

 it is better to fix a glass tube into a battery cell and to employ it 

 as the septum through which the gas is diffused. The following 

 experiment was also shown : — A porous battery cell was attached 

 to the short tube of a wash-bottle, both tubes being previously 

 turned upwards ; when a jar of hydrogen was placed over the 

 battery cell, the gas diffused through the cell, and the change of 

 pressure caused the water to issue like a fountain several feet 

 in height. I believe this arrangement was devised by Prof. 

 Bloxam. 



Now I must ask you to follow me a step further. In 1846 

 Mr. Graham read a paper before the Royal Society ' ' On the 

 Motion of Gases." He showed that the effusion of gases through 

 a minute hole in a platinum plug left no doubt of the truth of a 

 general law that Afferent gases pass through minute apertures in 

 times which are as the square roots of their respective specific 

 gravities, or with velocities which are inversely as the square 

 roots of their specific gravities ; or in other words, he experimen- 

 tally verified the mechanical law that the velocity with which a 



» Phil Trans., 1863. 



gas rushes into a vacuum through such an aperture, is the same 

 as that which a heavy body would acquire in falling from the 

 height of an atmosphere, composed of the gas in question, of 

 uniform density throughout The relative rates of effusion and 

 diffusion are alike, but Mr. Graham is careful to observe that 

 the phenomena are essentially different in their nature. The 

 former affects masses of gas, the latter (diffusion) only affects 

 molecules. 



The apparatus Mr. Graham employed consisted of two glass 

 jars ; the one containing the gas to be examined was placed in a 

 pneumatic.trough, and the other stood on the plate of an air- 

 pump. They were in connection, a series of tubes containing 

 the usual reagents for purifying and drying the gas being inter- 

 posed between them. The jar on the air-pump was exhausted, 

 and the gas entered it through a minute orifice in a platinum 

 disc, the rate of passage being observed by the aid of a mercurial 

 column. 



Three years later Mr. Graham published a pap;r giving the 

 results of an investigation on what he considered to be a funda- 

 mental property of the gaseous form of matter, which he termed 

 transpiration. He employed capillary tubes, and found that 

 effusion and transpiration differed widely ; ^ " for if the length of 

 the tube is progressively increased, and the passage for all gases 

 becomes greatly slower, the velocities of the different gases are 

 found to diverge rapidly from their effusion rates." The veloci- 

 ties at last, however, attain a particular ratio with a given length 

 of tube and resistance, and preserve the same relation to each 

 other for greater lengths and resistances, the most simple result 

 probably being that of hydrogen, which has exactly double the 

 transpiration rate of nitrogen, the relation of these gases as to 

 density being as I : 14. 



Note.— It is impossible to make the four columns strictly comparable on 

 account of the difference of the conditions under which the experiments 

 were made. 



Thus, in what are very nearly Mr. Graham's own words, a 

 gas may pass into a vacuum in three different modes ; that is, by 

 effusion, transpiration, or diffusion, and I hope you will bear 

 with me while I recapitulate them. 



1. The gas may enter the vacuum by effusion, that is, by 

 passing through a minute aperture in a thin plate, such as a 

 puncture in platinum-foil made by a fine steel point. The rela- 

 tive times of the effusion of gases in mass are similar to those 

 of the molecular diffusion, but a gas is usually carried by the 

 former kind of impulse with a velocity many thousand times as 

 great as is demonstrable by the latter. 



2. If the aperture of efflux becomes a tube, the effusion rates 

 are disturbed. The rates of flow of different gases, however, 

 assume again a constant ratio to each other when the capillary 

 tube is so elcmgated that the length exceeds the diameter by at 

 least 4,000 times. The transpiration rates appear to be inde- 

 pendent of th"e material of the capillary ; they are not governed 

 by specific gravity, and are indeed singularly unlike the rates of 

 effusion. The ratios appear to be in direct relation with no 

 other known property of the same gases, and they form a class 

 of phenomena remarkably isolated from all that is at present 

 known of gases. 



For instance it will be seen by the table already given that 

 the rate of carbonic acid which is low for effusion and diffusion, 

 becomes comparatively rapid when the gas passes by transpira- 

 tion. 



3. A plate of compressed graphite, although it appears 



' PhiJ. Trans., 1849, p. 349. 



