June 29, 191 ij 



NATURE 



585 



ordinary salts, they may be assumed to be permeable to 

 the sodium ion, while holding back the much larger organic 

 ion. On the other hand, it is found in actual experiments 

 that the sodium ion does not escape from the osmometer. 



Many collateral results show that the membrane does 

 not mechanically directly constrain the sodium ion from 

 diffusing. Thus a membrane, even when deeply impreg- 

 nated with Congo red, will permit the passage of sodium 

 sulphate. Biltz and von Vegesack,' in order to explain 

 the results, assume that the sodiufn ion of congo red, 

 because it can diffuse through the membrane (and in spite 

 of the fact that it actually does not do so), therefore con- 

 tributes nothing to the osmotic pressure. I find myself 

 quite unable to accept this view. 



Osmotic pressure is a measure of the total constraint 

 imposed on the system solution and pure solvent by a 

 membrane. To the organic ions, owing probably to their 

 size, the membrane offers what may be called a mechanical 

 constraint. The sodium ions, on which the membrane 

 exerts no such direct mechanical constraint, do not escape, 

 because they are held back by the electrostatic attraction 

 of the negatively charged organic ions. When equilibrium 

 is reached, the electrostatic pull inwards must balance the 

 osmotic pressure, which tends to drive the sodium ions 

 outwards. The total pressure borne by the membrane, 

 therefore, is the sum of the osmotic pressure of undis- 

 sociated molecules, of the organic ions, and of the sodium 

 ions held back by their attraction to the organic ions. 



On this view, the osmotic forces would produce at the 

 membrane an average orientation of plus and minus ions, 

 which should appear as a contact potential difference 

 between the solution and the solvent. The magnitude of 

 the electromotive force can be calculated in several ways, 

 of which the simplest is as follows. 



If E be the potential difference of the quasi-condenser 

 formed by the ions, P the osmotic pressure, and c the 

 concentcation of the dissociated salt in gr. equivalents 

 per c.cm., then the osmotic force acting outwards on one 



gr. equivalent of ions is - - , and the electrostatic pull in- 



^E 



c dx 



wards is -7-'/, where q is the charge on one gr. equiva- 

 lent. Since these balance one another, we have 



f/E^ I rfP 

 dx cq dx 



putting (; = P/RT we have 



a-E^RT^ 



dx tjP dx 



which on integration gives 



q ^ l\ 



This expression differs from Nernst's well-known equation 

 only in the absence of the terms representing the rate of 

 diffusion of the ions. 



From what is known of colloidal solutions, it is possible 

 that, except at a high dilution, some of the positive elec- 

 tricity might be carried by complexes too large to penetrate 

 the membrane. These would contribute either nothing at 

 all, or something less than the amount given by an equal 

 number of sodium ions, to the potential difference. Their 

 presence, therefore, would cause the calculated values to 

 exceed the observed values. W. B. Hardv. 



The Fox and the Fleas. 



In reference to the letters of Prof. Hughes in Nature 

 of March 23 and April 13, and his query as to whether 

 the device adopted by foxes for divesting themselves of 

 fleas is " instinctive," may I be permitted to give some 

 particulars of a similar practice which has been observed 

 in this country? 



Foxes were introduced into Victoria from England in 

 the early days of the colony, and are now plentiful tvi n 



1 Zeits. f. physik. Ch., 73. 481, 19>0- 



NO. 2174, VOL. 86] 



in the neighbourhood of Melbourne. Mr. P. R. H. St. 

 John, of the Botanic Gardens, tells me that, whilst 

 botanising or shooting in the neighbourhood of Point 

 Cook during the 'eighties, he has on at least a score of 

 occasions seen foxes enter the water with apparently the 

 same object as the fox observed by Mr. Day. 



Point Cook is situated on the west shore of Port Phillip 

 Bay, and is about twenty miles from Melbourne. The 

 surrounding country, being marshy, covered with saltbush, 

 and of little agricultural value, was practically uninhabited, 

 and a favourite hunting-ground for naturalists. 



The procedure adopted by the fox was to retreat slowly 

 into the shallow water (the beach being very level at that 

 spot) until only the head was visible, and then it would 

 disappear completely and rise to the surface about a yard 

 away, and, leaving the water with a bound, the fox would 

 rapidly reach the shore, shake itself like a dog, and make 

 off into the bush. The time occupied by the whole opera- 

 tion, which was only attempted when the water was quite 

 calm, would be about three or four minutes. 



Mr. St. John, though never closer than about fifty yards, 

 did not think that the foxes he observed held any wool or 

 fur in their mouth (there were no sheep grazing in the 

 locality), but he and his father, and various friends who 

 accompanied him, had come to the conclusion that the 

 object must have been to rid themselves of the fleas which 

 were always to be found on those specimens which they 

 shot. 



It will be noticed that this manoeuvre differs materially 

 from that of the English foxes in that no wool or fur 

 was used. This would suggest less call on the reasoning 

 power of the fox, but on the other hand the deficiency was 

 made up for by a final complete submersion and (there 

 being no current) a side movement and a rapid escape from 

 the water to dodge the dislodged and probably floating 

 fleas. 



The proverbial cunning of the race is surely to be seen 

 in this adaptation of method to the conditions to be found 

 on an uninhabited coast. Heber Green. 



Agricultural Chemistry Laboratory, The University 

 of Melbourne, May 23. 



Chemistry at the Forthcoming Meeting of the British 

 Association. 



May 1 direct attention to the following features in the 

 provisional programme of Section B (Chemistry) at the 

 British Association meeting in Portsmouth, beginning on 

 August 30 under the presidency of Sir William Ramsay? 



(a) Joint discussion with the section of agriculture on 

 the part played by enzymes in the economy of plants and 

 animals. . 



(b) Discussion on colloids (opened by Prof. Freundlich, 

 Leipzig, v^ith a contribution on the theory of colloids). 



(c) Discussion on indicators and colour. 



Many foreign chemists intend to bo present — amongst 

 others. Profs. Ostwald and Freundlich (Germany), 

 Wegscheider (Austria), Gautier, Haller (France), Clarke, 

 Barus (America), Righi (Italy), Pettersen, Euler (Sweden), 

 Birkelnnd (Norway), Zeeman, Cohen (Holland)— and it is 

 to be hoped that there will be a numerous attendance of 

 British chemists to do honour to these and other dis- 

 tinguished guests. James Walker. 



(President, Section B.) 



Edinburgh, June 24. 



Breath Figures. 



Lord Rayleigh's communir.-uion on hrr.nh fij^urfs 

 (Nature, May 25, p. 416) puts nn' in miml nl ;in cxpf-ri- 

 ment — if I may call it so — we made when we were 

 children, .'\fter breathing on a window-pane we wrote 

 our names on the glass with the point of a finger. Now 

 after having waited until the moist deposit had dis- 

 appeared, and again breathing on the glass, the written 

 characters became quite legible. 



This seems quite to agree with Lord Rayleigh's explana- 

 tion, grease on the fingers causing the phenomenon. 



Delft, June 6. J. W. Giltay. 



