588 



NATURE 



\pct. 1 6, 1879 



beginning of this year Prof. Osborne Reynolds gave, in a 

 remarkable paper,' the results of experimental researches 

 "On Thermal Transpiration of Gases through Porous 

 Plates," and showed the existence of a class of very 

 marked phenomena which had escaped the notice of 

 other observers. More recently Dr. John H. Long has 

 studied the diffusion of liquids,' starting from the work 

 of Graham, which must be regarded as the first and only 

 general investigation of this subject we possess. Dr. 

 Long's results will be briefly given in his own words, but 

 it may be well to draw attention, as he does, to a few 

 facts connected with the history of the subject. In two 

 papers read before the Royal Society in 1850, Graham 

 estabhshed that— 



1 . The velocity of diffusion is different for each'substance 

 in solution. 



2. The amounts of salt diffused in a given time from 

 solutions of the same substance, but different concentra- 

 tions, are very nearly proportional to the concentration. 



3. The amount of salt diffused from a given solution 

 increases with the temperature. 



Fick subsequently showed that liquid diffusion may be 

 compared to the conduction of heat, that is, the spread of 

 salt particles through water is in many respects analogous 

 to the spread of heat in a conducting body, and that 

 formula:, similar to those established by Fourier for the 

 latter case, may be applied in the former. 



Certain experimentalists then employed optical methods 

 of observation in determining the rate of diffusion of salts 

 in solution, but Stefan showed that the optical methods 

 "are based on a false assumption, and that they can 

 therefore give only false results." It will be sufficient to 

 state, however, that after a careful review of the work of 

 the several investigators, Dr. Long divides the researches 

 into two classes : — 



1. Those which are concerned with the physical side of 

 the question, that is, with the determination of "the 

 constant of diffusion " for a single substance. To this 

 class belong the researches of Fick, Simmler, and Wild, 

 Voit, Hoppe-Seyler, Johannisjanz, Weber, and Stefan. 



2. Those which treat the subject from a chemical point 

 of view, by comparing the rates of diffusion of many 

 different substances. To this class belong the investiga- 

 tions of Graham, Beilstein, and Marignac. Dr. Long 

 points out that " in regard to the first class it may be said 

 that a very satisfactory end has been attained. The 

 proof of Fick's law by Weber and Stefan, and the deter- 

 mination of the influence of temperature and concentration 

 of solution by the former, leave little to be desired in con- 

 nection with this part of the subject. The same cannot be 

 said, however, of the other, the chemical side." The 

 experiments of Beilstein are not sufficiently numerous to 

 establish much with certainty, and those of Graham and 

 Marignac, while agreeing well among themselves, do not 

 establish the dependence of diffusion on the molecular 

 weight or other physical property of the substance 

 employed. 



Facts such as these have led Dr. Long to undertake a 

 lengthy series of experiments, in which he employed a 

 method that renders it possible to determine the rate_ of 

 diffusion from hour to hour, and to insure that the diffusion 

 takes place into a medium whose concentration is always 

 zero. His apparatus may be roughly described as consisting 

 of a U-tube placed in a'beaker, which contains the solution 

 to be investigated. The ends of the tube are bent over the 

 beaker, one end being connected with a funnel into which 

 water slowly drops, displacing the solution in the U-tube, 

 which flows out from the other end at about the rate of 

 40 cc. in an hour. The base of the U-tube is open, and 

 is connected with a short vertical tube whose internal 

 diameter is 15 mm. This larger tube is open only at the 

 bottom, and is arranged concentrically with the beaker. 



' Proc. Roy. Soc, iBjp, p. 304. 



" A dissertation presented to the F.-icuUy of Science of the University ot 

 Tilbingen, 1879. 



Diffusion thus takes place between the solution below the 

 line of junction of the short tube with the U-tube and the 

 water contained in the latter, the diffused particles being 

 carried away and discharged ; in other words, there is 

 diffusion between a level of constant concentration and 

 a level " of concentration zero." Space will only per- 

 mit us to notice the general conclusions at which Dr. 

 Long has arrired. He observes that no simple rela- 

 tion is recognisable between dift'usion and other physical 

 phenomena if we merely state the results in grammes of 

 substance diffused. If, on the other hand, the results are 

 stated as the numbers of molecules diffused, several in- 

 teresting relations appear. For instance, it can be shown 

 that the chlorides, bromides, and iodides of the alkaline 

 metals form a series in which NH4 stands between K and 

 Na ; and in this series the chloride, bromide, iodide, and 

 cyanide of potassium have nearly the same rate. The 

 chlorides of the dyad metals Ba, Sr, Cr, and Mg are 

 also seen to form a series as to their rates of diffusion. 

 It can further be shown, by comparing Kohlrausch's 

 results on the electrical conducting power of liquids with 

 the diffusion rates, that those salts which in solution offer 

 the least resistance to the passage of the galvanic current 

 are the ones which diffuse most rapidly. In most cases 

 it appears that the salts having the greatest molecular 

 volume diffuse the best, and those salts which absorb the 

 greatest amount of heat on passing into solution are also 

 the ones which diffuse most rapidly. 



Dr. Long shows that Graham's view that no relaticn 

 exists between the molecular weight and the rate of dif- 

 fusion requires modification, for the alkaline chlorides, 

 bromides, and iodides stand in the same order as regards 

 molecular volume, rate of diffusion, conducting power, 

 and latent heat of solution. 



In conclusion Dr. Long indicates the direction in 

 which he proposes to continue the research, which, we 

 may add, bears evidence of being the work of an 

 able physicist, from whom many valuable researches may 

 be expected. W. Chandler Roberts 



THE PARKES MUSEUM OF HYGIENE 



THERE is one all important matter with which neither 

 the great Institution in Bloomsbury nor that at South 

 Kensington has virtually any concern ; this important 

 matter is Hygiene, the knowledge and application of the 

 laws of life, which in so far as they are perfect banish 

 disease from the human race. The Parkes Museum of 

 Hygiene has been formed to promote this department of 

 the numerous applications of science. 



In the spring of 1876 the movement for the formation 

 of the Parkes Museum commenced. It was the outcome 

 of a very general desire to perpetuate in some useful way 

 the memory of the late Dr. Parkes, whose life had been 

 so unselfishly spent in promoting the welfare and happi- 

 ness of his fellows by extending the knowledge of the 

 laws of health, and whose untiring energy and keen 

 intellect did such good service in clearing away the ignor- 

 ance and superstition which accepted disease as the 

 inevitable accompaniment of human life in this world. 

 The movement rapidly developed into shape, and finally 

 the Parkes Museum of Hygiene was opened to the public 

 in June last, with a fairly representative collection of 

 mechanical appliances, models, plans, and books, designed 

 to promote health, of which a descriptive and illustrated 

 catalogue was published. Since then so many valuable 

 additions have been made to the museum that an enlarged 

 and improved catalogue has been issued. The affairs of 

 the museum are administered by an executive committee 

 of which Sir William Jenner is chairman, and at present 

 the cost of maintaining the museum has to be met wholly 

 by voluntary contributions. The collection of appliances, 

 models, &c., is temporarilv located in the largest room of 

 the south wing of University College, which, together 



