376 



NA TURE 



[August 15, 1901 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to eo> respond with the writers of, rejccei 

 manuscripts intended for this or any other part of Natuke. 

 No notice is taken of anonymous communications.'^ 



Pearl and Pearl-shell Fisheries. 



In connection with Sir West Ridgeway's anxiety, as Governor 

 of Ceylon, to revive the pearl fishery off the north-west coast of 

 the island, and the appointment by the Secretary of State for the 

 Colonies of so able a zoologist as Prof. Herdman to report on 

 the subject — so classic to zoologists since Dr. Kelaart's paper 

 and the display of fine examples of the pearl shells by the Indian 

 Government in the London Fisheries' Exhibition of 1S83 — it 

 may be interesting to mention the activity of the Queensland 

 Government in this and allied subjects. Besides the work of 

 Mr. Saville Kent and the recent (private) investigations of Mr. 

 Lyster Jameson, the Queensland Government early last year 

 appointed an able young zoologist, Mr. James R. Tosh, to make 

 investigations on the life-history of the species which produces 

 the peatl-shells of commerce, the formation and growth of pearls, 

 and other questions bearing on the pearl fishery. He is now 

 busy on Thursday Island. Moreover, Mr. Tosh informs me 

 that the (Queensland Government has just sanctioned a grant of 

 1500/. for the erection of a marine laboratory on a small island 

 about two miles distant (from Thursday Island), and in the 

 centre of the pearl-fishing grounds, though at some distance from 

 the coral area. This laboratory will have, besides the work- 

 room and quarters for Mr. Tosh and his staff, three concrete 

 tanks for experimental work. W. C. McIntosh. 



Barham, Springfield, Fife. 



A Possible Method of Attaining the Absolute Zero of 

 Temperature. 



In your issue of July 25 there appeared an interesting article 

 on the liquefaction of gases. It was shown that by rapidly 

 evaporating hydrogen. Prof. Dewar obtained a temperature of 

 13-15° C. (absolute). By a similar use of the more volatile helium, 

 probably an even lower temperature could be obtained. 



But, as the author pointed out, such methods will not enable 

 us to reach the absolute zero itself. 



May I be allowed to suggest that thermoelectric phenomena 

 will be of some use in attaining the desired result ? 



Peltier showed that if a current be passed across an antimony- 

 bismuth junction, in one direction heat is developed and in the 

 reverse heat is absorbed and an appreciable cooling effect 

 obtained. 



Similarly, in the case of any two other metals, heat is 

 generated if the current traverses the surface of contact in one 

 direction and is consumed if it passes in the opposite direction — 

 the quantity of reversible heat being in each case proportional to 

 the strength of the current and to a coefficient it, depending on 

 the nature of the metals and their temperature. 



So that, in general, if r is the resistance of the part of the 

 circuit containing the junction, the energy converted into 

 frictional heat is Q-rt and the energy converted into reversible 

 heat is Cir. 



Hence, if H be the quantity of heat produced in / seconds 

 we have : — 



J. H = Cir/ + C-rt. 



By making a small hole at the junction of a bismuth and 

 antimony bar, in which was placed a drop of water and a small 

 thermometer, the whole being cooled to zero, Lenz found that 

 when a current was passed from bismuth to antimony the water 

 was frozen and the thermometer sank to -3"5° C. 



Opposing this fall in the junction's temperature there are, in 

 general, two influences. Firstly, when a current is passed 

 through a conductor a frictional generation of heat occurs, 

 which tends to mask the cooling efiect. .Secondly, when one 

 part of a circuit is at a much lower temperature than the other 

 parts, heat will flow by conduction from the hotter to the colder 

 parts and thus again oppose further cooling. 



When a stationary low temperature has been reached by the 

 junction, we must suppose that as much heat is absorljed by the 

 current in unit time as is imparted to the junction by means of 

 both of those influences I have mentioned working together. 



NO. 1659, VOL. 64] 



If, therefore, we could diminish or do away with these, a 

 very great cooling effect could be obtained. 



The frictional heating effect could be eliminated to a great 

 extent by cooling the whole to the lowest temperature attainable 

 by the use of liquid hydrogen. Recent experiments by Profs. 

 Fleming and Dewar show that an astonishing fall in the specific 

 resistance of most metals takes place at very low temperatures. 

 Thus the specific resistances of copper and iron fall from 1564 

 and 9115 respectively ato'C. to 2Sq and 1220 at ~ 200" C, while 

 at a temperature only 20° C. lower, these numbers become 144 

 and 660 — i.e. the specific resistance at - 220° C. is actually half 

 that at -200° C. (vide Foster and .Atkinson's " Electricity and 

 Magnetism," p. 162, iSg6ed.) Such an enormous diminution 

 in the specific resistance leads one to expect that at only 13-14° C. 

 from the absolute zero — the lowest temperature yet attained by 

 Dewar — the resistance would be practically negligeable, so that 

 the term C-rt in the above expression would become extremely 

 small even when currents are employed considerably more 

 powerful than those which can be used at ordinary temperatures 

 for producing the Peltier effect. 



If, then, 7r remained appreciably large, it is quit; possible that 

 matter could by such means be chilled almost to the absolute 

 zero without the masking effect of frictional heat becoming 

 sensible. 



The second influence, namely, the flow of heat by conduction 

 from the hotter parts of the bar to the cold junction, could be 

 eliminated by avoiding a sensible temperature difference between 

 the chilled junction and the rest of the circuit. For instance, 

 each small section of the main circuit could be cooled simul- 

 taneously vvith the junction by means of a number of other 

 chilled thermoelectric junctions. By the use of some such con 

 trivance, the temperature of the junction need never become 

 very much lower than that of the rest of the ciicuit. 



■The coeflicient tt would certainly alter with the temperature ; 

 unless it completely vanishes for all bodies at very low tem- 

 peratures, such an effect could be corrected by suitably choosing 

 the metals forming the junctions. Geoffrey Martin. 



Bristol, July 26. 



Food of the Senegal Galago. 



The following facts may interest some of your readers as 

 pointing out the possibility of a rare tropical animal being able 

 to maintain itself unaided for some weeks in an English country 

 town . 



On the evening of June 20 an African galago [G. senegalensis) 

 escaped from my laboratory in Eton. For some litile time it 

 was not seen or heard, but after that it constantly made its 

 appearance in gardens, on house roofs, &c., until, on the night 

 of July 28, it w-as caught while rifling a cupboard. Previous to 

 this date it had never been seen inside a house, so that how it 

 managed to obtain food is somewhat of a mystery. Probably 

 it lived on fledglings which it took out of the nest, and later on 

 the decrease in their numbers forced it to forage for less tempting 

 prey. Its strictly nocturnal habits and great agility no doubt 

 preserved it from being destroyed by dogs. M. O. Hilt.. 



Pseudoscopic Vision without a Pseudoscope. 



The curious optical illusion which has been noticed by Prof. 

 R. W. Wood and described by him in Nature for August S 

 under the heading of " A New Optical Illusion " has been 

 known for many years. 



It is mentioned in Helmholtz's great work on physiological 

 optics in the chapter on the stereoscope and pseudoscope. It 

 appears to have been first described by Prof. Joseph Le Conte 

 in 1S69 (see Silliman's American Journal of Science for January 

 1S69 and Phil. Mag. February 1S69). 



Both these authorities mention a further similar illusion not 

 described by Prof. Wood, which I think is a more striking 

 illusion. If one looks at a pattern of which the distance 

 between the centres of contiguous figures is somewhat less than 

 the distance between the two eyes, and if we gaze at it in such 

 a manner as if we were looking at a distant object beyond it, we 

 then get the illusion of a much increased pattern at a consider- 

 ably greater distance from the eye. -A. S. Davis. 



Roundhay, Leeds, August 9. 



