NA TURE 



[July 5, 1906 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Osmotic Pressure. 



In the issue of Nature for May 17 (p. 54) appears a 

 communication by Mr. Wfietham in which he attempts to 

 consign actual experimental work on osmotic pressure 

 to the humble role of showing how far the assumptions 

 made in so-called thermodynamical proofs can be realised 

 e.vperimentally. Among other things, the attempts to 

 apply thermodynamic reasoning to osmotic processes involve 

 the assumption of a membrane which is semi-permeable 

 and which at the same time is quite passive, that is to 

 say, which shows no selective action. Now in my paper 

 (referred to in Nature for May 3, p. 19) I have demon- 

 strated conclusively by experiment that in actual osmotic 

 processes the selective influence of the membrane is always 

 present, and is the determining factor as to whether 

 osmosis will take place at all, and, if so, in what direc- 

 tion. In studying that paper, the reader will also see 

 that the more nearly a membrane is semi-permeable in 

 character in practice, the greater is its selective action. 

 Ill fact, it is the pronounced selective action of the n^em- 

 hrane which makes it approximately semi-permeable. This 

 being the case, it is evident at once that thermodynamic 

 reasoning cannot be applied to actual osmotic processes, 

 and that the experimental work on osmotic pressure does 

 not play that humble rdle to which Mr. Whetham would 

 consign it. 



Mr. Whetham sees perfect semi-permeable membranes 

 (i) in the surface of growing crystals of pure solvent which 

 separate from a solution when it freezes, and (2) in the 

 free surface of a solution of a non-volatile solute as it 

 evaporates, and states that " from these two facts follows 

 the validity of the thermodynamic relations between 

 osmotic pressure on the one side and freezing point and 

 vapour pressure on the other." Now I must insist that 

 the formation of crystals from a solution, or the concen- 

 tration of a solution by evaporation, are not osmotic 

 processes. There are, in fact, no actual membranes or 

 septa involved in these processes, and to regard them as 

 *' osmotic " in character only causes much confusion, for 

 thev have nothing in common with an actual osmotic 

 process, in which a membrane — an additional phase with 

 specific selective action — is always present as a determining 

 factor. 



In how far it is allowable to apply thermodynamic reason- 

 ing to the evaporation of a solution or the formation of 

 crystals from a solution I shall not attempt to discuss 

 here, for it is quite outside the main subject with which 

 my paper deals, namely, the nature of osmosis and osmotic 

 pressure. For the same reason I shall not enter upon a 

 discussion of Mr. Whetham's contention that the theory 

 of electrolytic dissociation " rests upon electrical evidence, 

 :ind by such evidence it must be tried." In this connec- 

 tion it may suffice to refer the reader to the paper which 

 1 have prepared at the request of the Faraday Society (see 

 Trans. Faraday Soc, vol. i., also Phil. Mag. for February, 

 1005), in which I have directed attention to the fact that, 

 in creating the theory of electrolytic dissociation, the actual 

 phenomena of electrolysis have played a minor part. 



Concerning the remarks made in Nature of May 17 

 (p. 54) by Lord Berkeley and Mr. Hartley, I should like 

 to state that, so far as I am aware, the only direct measure- 

 ments of osmotic pressure which thev have made are some 

 preliminary results published in vol. Ixxiii. Proc. Roy. 

 Soc, pp. 436-443. In their article in vol. Ixxvii. Proc. 

 Roy. Soc, p. 156, I find no direct measurements of osmotic 

 pressure, but simply results of vapour-tension measure- 

 ments from which osmotic pressures have been computed 

 bv means of a modification of a formula of Arrhenius. 

 Of the results given in the two papers mentioned, there 

 is but one case that is comparable, namely, that at con- 

 centration 420 grams sugar per litre, the other deter- 

 minations having been made at different concentrations, 

 so that they are not comparable. Furthermore, all 



VO. 19 14, VOL. 74] 



their direct osmotic-pressure measurements were made 

 without stirring, and they are consequently not at all 

 final. I have also in my paper directed attention to the 

 fact that copper ferrocyanide membranes imbedded in 

 porous porcelain are particularly unsuitable for making 

 conclusive direct measurements of osmotic pressure. In 

 these circumstances, it appears that their claim that they 

 have shown experimentally that aqueous' solutions of cane 

 sugar give the same osmotic pressure, whether observed 

 directly or deduced indirectly from their vapour-pressures, is 

 not well founded. 



As to the computation which Lord Berkeley and Mr. 

 Hartley make concerning one of my experiments, I would 

 state that they assume as a basis for their calculation that 

 the slight amount of sugar found in the outer liquid occurs 

 there because the solution, as such, has passed through 

 the septum. Now this assumption is entirely untenable in 

 the light of the numerous experiments given in my paper 

 illustrating the nature of the osmotic process, and their 

 criticism is consequently worthless. Louis KAHLE.NnERG. 



University of Wisconsin, Madison, June 15. 



The Olfactory Sense in Apteryx. 



About a year ago I stated in your columns (May 18, 

 1905, p. 64) that I was trying to have experiments carried 

 out with the object of ascertaining whether the olfactory 

 sense of the kiwi is perceptibly developed, as one would 

 suppose it to be from certain structural peculiarities in 

 which the bird is unique, viz. the great relative size of 

 the olfactory lobes of the brain and the great size of the 

 olfactory capsule as seen in the skull. 



I wrote to the curators of Little Barrier and Resolution 

 Islands, which are reserved as sanctuaries for birds, ask- 

 ing each of them to try certain experiments for me with 

 the object, first, of finding out whether the kiwi exhibited 

 any preference for particular species of earthworm, and, if 

 so, whether any difference in odour, or noticeable differ- 

 ence in colour, was perceptible to them (the curators). I 

 asked whether it was possible to deceive the kiwi in any 

 way by appealing to its sense of smell, while excluding 

 those of sight, hearing, and touch, and formulated a few 

 simple experiments with this end in view. 



I recently received a reply from the curator of 

 Resolution Island, in Dusky Sound, who is a careful 

 observer of the habits of birds. Mr. Richard Henry ex- 

 perimented with the larger South Island bird, .Ipferyx 

 australis, usually termed the roa-roa, in opposition to the 

 other South Island bird, the small grey kiwi, A. oweni. 

 The former feeds chiefly on earthworms, the latter on 

 grubs of various kinds. Mr. Henry placed a number of 

 earthworms at the bottom of shallow buckets and covered 

 them with four inches of earth. When such a bucket was 

 placed on the ground the roa got quite excited in its hunt 

 through the earth, probing to the bottom for the worms. 

 It must be borne in mind that, according to several good 

 observers, the roa (and kiwi) is practically blind during 

 the day time, and, moreover, the bunch of hair-like 

 feathers at the base of its snout intervenes between its 

 eyes and the ground in this operation, while Mr. Henry 

 states that it makes such a " sniffling noise " that it would 

 be unable to hear a worm, even if the latter made any 

 disturbance in the soil. There remains, therefore, the 

 possibility that the tip of the beak is highly sensitive, and 

 that it finds the worms by touch. 



But Mr. Henry writes that the bird seemed readily to 

 be aware whether worms were below the earth without 

 touching the .soil, for " when I put down a bucket of 

 earth without worms in it, the bird would not even try 

 it; but the moment a bucket containing worms (covered 

 with earth) was put down the roa was full of interest in 

 it," and commenced to probe at once with its long beak. 



Further, Mr. Henry took several dead worms that had 

 been severely pressed by the spade in digging them up the 

 previous day, and put them at the bottom of a bucket of 

 earth, and at the end of half an hour the roa had not left 

 a scrap of worm behind. He tried the roa with a bucket 

 of earth that had been searched by it on the preceding day, 

 but the bird " would not even look at it." Then he placed 

 a couple of worms under the earth at the bottom of the 

 bucket, and again allowed the roa to have access to it ; 



