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NATURE 



{May II, 1876 



papers by Prof. Carey Foster on " Magnetic Apparatus " 

 and " Electrical Instruments ;" a paper by Mr. J. Norman 

 Lockyer on " Astronomical Instruments ; " by Prof. Good- 

 eve on " Applied Mechanics," by Prof. McLeod on 

 " Chemical Apparatus and Products," by Mr. R. H. Scott 

 on " Meteorological Instruments." " Geographical In- 

 struments and Maps" are illustrated historically and 

 descriptively in four papers by Mr. C. R. Markham, 

 and one by Capt. J. E. Davis. Prof. Geikie treats of 

 "Geology," Mr. Warington Smyth of "Apparatus used 

 in Mining," Prof. Story Maskelyne of " Crystallography 

 and Mineralogy," Prof. Huxley of " Instruments employed 

 in Biological Research," and Mr. H. C. Sorby of " Micro- 

 scopes." Is not this strong enough evidence of the 

 genuine interest which British men of science take in 

 this Loan Collection of Scientific Apparatus ? 



There is only one drawback to our joy in seeing this 

 collection at last completed and ready to be thrown 

 open to the public : it is after all only a " loan " collec- 

 tion, and in a few months must be disorganised, and the 

 apparatus returned to their^owners. We have some reason 

 to hope, however, that this will not be the end of all the 

 labours of the eminent men who have exerted themselves 

 to make the collection a success ; we are persuaded that 

 in time it will be succeeded by a permanent collection, 

 which will form a Science Museum on an equal footing 

 with the other Museums supported by Government. The 

 Introduction to the Handbook says : — 



" The Lord-President of the Council, the Duke of 

 Richmond, and the Vice-President, Viscount Sandon, in 

 explaining the objects of the collection, took occasion to 

 refer to the recommendations of the Royal Commission 

 on Scientific Instruction, with regard to the creation of a 

 Science Museum. Their Lordships stated their convic- 

 tion that the development of the Educational and certain 

 other Departments of the South Kensington Musf^um, 

 and their enlargement into a Museum somewhat of the 

 nature of the Conservatoire des Arts et Metiers in Paris, 

 and other similar institutions on the Continent, would 

 tend to the advancement of science, and be of great ser- 

 vice to the industrial progress of this country." 



We cannot doubt that neither Government nor the 

 public, after having substantial evidence of the value and 

 important results of a Science Museum in this Loan Col- 

 lection, will rest satisfied until this country is at least on 

 an equal footing in this respect with our neighbour 

 France. It stems to us that a permanent Science Museum 

 will be the natural outcome of the unexpectedly magnifi- 

 cent collection which the Queen will open on Saturday ; it 

 cannot fail to make the public at large conscious of a 

 serious want which for long has been painfully felt by men 

 engaged in scientific research, both pure and applied. 



DIFFUSION OF GASES THROUGH ABSORB- 

 ING SUBSTANCES 

 Ueber die Diffusion der Gase durch absorbirende Sitb- 

 stanzen. Habilitationsschrift der Mathematischen 

 und Naturwissenschaftlichen Facultat der Universitat 

 Strassburg, vorgelegt von Dr. Sigmund v. Wroblewski, 

 erstem Assistanten am physikalischen Institute. (Strass- 

 burg : G. Fischbach, 1876.) 

 THE importance of the exact study of the motions of 

 gases, not only as a method of distinguishing one 

 gas from another, but as likely to increase our knowledge 



of the dynamical theory of gases, was pointed out by 

 Thomas Graham. Graham himself studied the most 

 important phenomena, and distinguished from each other 

 those in which the principal effect is due to different pro- 

 perties of gases. 



The motion of large masses of the gas approximates ! 

 to that of a perfect fluid having the same density and 

 pressure as the gas. This is the case with the motion of 

 a single gas when it flows through a large hole in a thin 

 plate from one vessel into another in which the pressure 

 is less. The result in this case is found to be in ac- 

 cordance with the principles of the dynamics of fluids. 

 This was approximately established by Graham, and the 

 more accurate formula, in which the thermodynamic pro- 

 perties of the gas are taken into account, has been verified 

 by the experiments of Joule and Thomson. (Proc. R. S., 

 May, 1856.) 



When the orifice is exceedingly small, it appears from 

 the molecular theory of gases that the total discharge 

 may be calculated by supposing that there are two currents 

 in opposite directions, the quantity flowing in each current 

 being the same as if it had been discharged into a vacuum. 



For different gases the volume discharged in a given 

 time, reduced to standard pressure and ^temperature, is 

 proportional to — 



where p is the actual pressure, s is the specific gravity, 

 and 6 the temperature reckoned from — 274° C. 



When the gases in the two vessels are different, each 

 gas is discharged according to this law independently of 

 the other. 



These phenomena, however, can be observed only when 

 the thickness of the plate and the diameter of the aperture 

 are very small. 



When this is the case, the distance is very small between 

 a point in the first vessel where the mixed gas has a cer- 

 tain composition, and a point in the second vessel where 

 the mixed gas has a quite different composition, so that 

 the velocity of diffusion through the hole between these 

 two points is large compared with the velocity of flow of 

 the mixed gas arising from the difference of the total 

 pressures in the two vessels. 



When the hole is of sensible magnitude this distance 

 is larger, because the region of mixed gases extends 

 further from the hole, and the effects of diffusion become 

 completely masked by the effect of the current of the gas 

 in mass, arising from the difference of the total pressures in 

 the two vessels. In this latter case the discharge depends 

 only on the nature of the gas in the vessel of greater 

 pressure, and on the resultant pressures in the two 

 vessels. It consists entirely of the gas of the first vessel, 

 and there is no appreciable counter current of the gas of 

 the other vessel. 



Hence the experiments on the double current must be 

 made either through a single very small aperture, as in 

 Graham's first experiment with a glass vessel accidentally 

 cracked, or through a great number of apertures, as in 

 Graham's later experiments with porous septa of plaster 

 of Paris or of plumbago. 



With such septa the following phenomena are ob- 

 served : — 



When the gases on the two sides of the septum are 



