68 



NATURE 



[March i6, 1916 



for considerable periods of time, as is often necessary 

 when using X-rays for therapeutic purposes. 



The adaptation of tungsten for this purpose is an 

 example of the great value that lies hidden in the 

 rare and little-known elements, and doubtless other 

 instances of a similar nature will develop as the metals 

 become available. ' 



OSMOTIC PRESSURE OR OSMOTIC 

 SUCTION? 



IT has often been assumed that van't Hoff's dis- 

 covery, that the simple gas-law, PV = RT, may 

 be applied to the osmotic pressures of dilute solutions, 

 justifies the view that osmotic pressure is caused by 

 the bombardment of a semi-permeable membrane by 

 the molecules of the solute, just as gas-pressure is 

 caused by the bombardment of the containing vessel 

 by rapidly moving gas-molecules. A recent exposition 

 of this view by Prof. Ehrenfest, in the Proceedings of 

 the Amsterdam Academy (vol. xvii., pp. 1241-1245), 

 has elicited a reply from Prof. J. J. van Laar {ibid., 

 vol. xviii., pp. 184-190), which will be read with 

 very great interest by all those who have seen in the 

 mechanism of osmosis an even more difficult problem 

 than that of expressing the magnitude of the osmotic 

 pressure by means of a mathematical formula. Prof, 

 van Laar's reply is of exceptional value in that it 

 demonstrates the inadequacy of the gas-analogy from 

 the thermodynamic point of view, and so challenges 

 the simple kinetic theory of osmosis on what has 

 generally been supposed to be its strongest ground. 



The osmotic pressure may be expressed, according 

 to Van Laar, by the equation, 



P = RT/T»,{-log(i-x) + ax'}, 



where x is the molecular concentration of the dissolved 

 substance, and a is an " influencing " coefficient, which 

 expresses the consequences of the interaction of the 

 molecules of the solvent with those of the dissolved 

 substance. The logarithmic term is an essential 

 feature of the thermodynamic equation, and it is urged 

 that all kinetic theories which lead to expressions with- 

 out a logarithmic member must be rejected. 



The thermodynamic equation, it is true, leads to an 

 expression for dilute solutions which is identical with 

 that of van't Hoff. But in practice it is found that 

 in more concentrated solutions deviations appear which 

 are much smaller than those for non-ideal gases. We 

 may therefore surmise that the so-called osmotic pres- 

 sure has an entirely different ground from that sug- 

 gested by van't Hotf's application of the gas-equation, 

 and that there is here no close relation but merely an 

 analogy. 



If the osmotic pressure were actually caused by the 

 pressure of the dissolved substance, as Ehrenfest, re- 

 viving the old theory, suggests, the pressure of the 

 sugar molecules against the semi-permeable membrane 

 would, in van Laar's opinion, cause the reverse effect 

 to that which is actually observed. No water would 

 pass from the pure solvent through the membrane 

 into the solution, giving rise to a hydrostatic pressure 

 in the osmometer; but, on the contrary, the inward 

 flow of water would be checked, since the pressure in 

 the solution would from the outset be greater than in 

 pure water. In reality, osmotic pressure is caused by 

 the water which penetrates through the semi-permeable 

 membrane, giving rise to a hydrostatic pressure which 

 prevents the further intrusion of the water. This ex- 

 cess of pressure is the so-called "osmotic pressure" of 

 the solution. 



Generally speaking, every theory which seeks to 

 interpret osmotic pressure kinetically must be based 

 on the diffusion of the water molecules on the two 



NO. 2420, VOL. 97] 



sides of the membrane. If this is done, the logarithmic 

 member arises of its own accord, and finds a place in 

 the equation, whether there is interaction between sol- 

 vent or solute or not, i.e. the a-term appears quite 

 independently of the logarithmic term. In van Laar's 

 opinion, the kinetic interpretation of osmotic pressure, 

 which is always reappearing again in new forms, is 

 moving, and has moved, in a wrong direction, and 

 should again be founded on the simple diffusion 

 phenomenon. T. M. L. 



POST-GRADUATE SCHOLARSHIPS AND 

 FELLOWSHIPS. 



'X'HE new list of scholarships and fellowships offered 

 *■ by the Leeds University has just been issued. It 

 includes some twelve entrance scholarships in arts, 

 science, medicine, and technology, awarded on the 

 results of the matriculation examination of the Joint 

 Matriculation Board, in addition to a certain number 

 (not specified) given by the local education authority. 

 There are also twelve Clothworkers' free studentships 

 in the textile department, and a " William Cooke " 

 scholarship in mining, determined by special examina- 

 tion or selection. In addition to the above are a num- 

 ber of senior scholarships, awarded to students of 

 special merit in the University, by the University, the 

 Leeds City Council, and by various donors who have 

 wished to perf>etuate with their names their interest 

 in the University. Such are the Leighton exhibitions 

 established by the trustees of Mrs. Isabel Leighton, of 

 Leeds, the Salt scholarship given by .Sir Titus Salt, 

 the John Rutson scholarship, and the Gilchrist student- 

 ship in modern languages. The list of post-graduate 

 scholarships and fellowships is a very meagre one. 

 There is one 1851 exhibition scholarship of 150/. 

 tenable for two years, and a number of 1851 exhibition 

 industrial bursaries of looZ., both awarded by the 

 185 1 Exhibition Commissioners, the first in science 

 and the second in some branch of technology. There 

 is, further, a research scholarship in colour chemistry 

 founded by the Clothworkers, and a scholarship in gas 

 engineering endowed by Sir Corbet Woodall. There 

 are also two scholarships in the faculty of medicine. 

 A limited number of research fellowships are also 

 awarded by the University to distinguished gradual* - 

 there is one in connection with the fuel departni< 

 in gas research founded by the Institute of G :- 

 Engineers, and one in colour and textile chemistn.-. 



It is generally recognised by university teachers thar 

 the year or years immediately following graduation 

 are in a sense the critical years of a student's career. 

 In science more especially he has laid up a fund of 

 knowledge which he is about to turn to practical 

 account. He has collected a store of potential energy ; 

 he has played the rdle of an "accumulator" during his 

 university course, and his energy is now to be turned to 

 usefuLwork. In the northern universities at least the 

 graduate has to earn his living, and whilst he is on 

 the look-out for congenial, as well as remunerative, 

 occupation he may often have to wait for many 

 months. It is at this critical time that a post-graduate 

 scholarship, sufficient for the student to keep himself 

 and release his parents from the burden of further 

 maintenance, is invaluable. It is invaluable not 

 merelv because it gives him time to look round and 

 relieves him from the necessity of accepting the first 

 vacancy that offers; but because he is learning in that 

 excellent school of research how to use his knowledge 

 and more especially how to depend upon himself. 



In the "Scheme for the Organisation and Develop- 

 ment of Scientific and Industrial Research " issued by 

 the Board of Education we have the promise of a large 

 extension of post-graduate research studentships and 



