40 



NATURE 



[September 8, 192 1 



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.] 



The Apparatus of Dr. Russ. 



I HOPE Dr. Hartridge will pardon my suggestion 

 that he is dismissing the possible effects of tempera- 

 ture too lightly. 



In the Phil. Trans. Roy. Soc. of 1792, p. 86, Mr. 

 Bennett described apparatus of great sensibility, in 

 which a piece of dragon-fly wing or thistledown 

 carried on a light arm suspended by a spider line in 

 a closed case responded with amazing sensibility to 

 the heat from a person at a distance in virtue of con- 

 vection currents set up by the warmer side of the 

 case. It is not surprising that suggestions of animal 

 magnetism should have been made, e.g. that the 

 right hand should act oppositely to the left, but the 

 author of the paper ignored these, and was content 

 with explanations based upon known laws of physics. 



A little later (1798) Cavendish in his famous paper 

 on the mean density of the earth showed how potent 

 minute differences of temperature were to disturb 

 even the 2-in. balls of lead that he suspended from 

 the ends of his lever. 



In 1862 Joule described in the Proc. Lit. and Phil. 

 Soc. Manchester, p. 73, a convection thermometer in 

 which a glass tube 2 ft. long and 4 in. in diameter 

 was divided longitudinally into two portions bv a dia- 

 metrical partition extending to within about i in. 

 of the top and bottom. " In the top space a bit of 

 magnetised sewing-needle, furnished with a glass 

 index, is suspended by a single filament of silk." 

 The draught up on the warmer side and down on the 

 cooler side caused the needle to be deflected, acting 

 on the glass index as a wind-vane. This was found 

 to be a superlatively delicate radiation thermometer. 



In 1890, in conjunction with the late Dr. Watson 

 and Mr. Briscoe, I showed to the Physical Society 

 (Phil. Mag., 1891, p. 59) an experiment which in- 

 creased the delicacy of the Joule thermometer very 

 greatly by replacing the straw and silk by a mirror 

 and counterweight hung by a quartz fibre, but we 

 found that by no system of screening, even in a cellar, 

 could we maintain such quiet in the air as to allow 

 the mirror to remain anywhere near the neutral posi- 

 tion or to remain at rest. One side or the other w^as 

 the hotter, and this was always changing. If we had 

 never succeeded in obtaining a real state of rest the 

 delicacy would have been useless. We, however, hit 

 upon a plan which did keep the two sides strictly alike 

 in temperature. We surrounded the tube bv an ex- 

 terior glass tube kept turning on its axis rapidly by 

 clockwork all day. As the exterior glass was opaque 

 to "dark heat " and no light was allowed to fall on 

 the tube, the inner tube could not have one side hotter 

 than the other, and then the mirror came to its 

 neutral position and was very fairly steady there, so 

 that heat developed electrically on one side of the 

 partition in warming the air gave rise to deflections 

 which could be measured with some certaintv. 



In all the delicate work that I have done with 

 quartz-fibre suspensions the strictest attention to 

 freedom from disturbance by air movement was 

 essential to success. Onlv by such special care can 

 air movements of so small an amount as 1 in. in a 

 fortnight'or so be avoided, and if not avoided a stable 

 zero on which everything must depend is impossible. 



NO. 27065 VOL. 108I 



In no ordinary large apparatus of the physical 

 laboratory is the air ever quiet, and in the closed box 

 of Dr. Russ is it safe to suppose that there are not 

 convection currents abundantly able to cause deflec- 

 tion of anything suspended by a single silk fibre? 



C. V. Boys. 



The Designation of the Radium Equivalent. 



In all problems that are primarily concerned with 

 strictly radio-active phenomena the quantity AN^ 

 denoting the number of atoms transformed in a unit 

 of time, plays a very important part. In such 

 problems comparable amounts of different radio- 

 elements are such as correspond to the same value of 

 AN. There is need for a name to denote the amount 

 of any radio-element, irrespective of family, that is 

 thus comparable to one gram of radium. If, tenta- 

 tively, we use the letter r to denote this desired name, 

 then an r of any inaterial may be defined as that 

 amount of the material that will produce transformed 

 atoms at the same rate as transformed atoms are 

 produced by one gram of radium. The quantity r 

 plays in radio-activity a part that is analogous to 

 that played by the gram-molecule in physical chemis- 

 try, and the advantages to be secured by naming it 

 are quite similar to those that 'were secured by the 

 introduction of the term "gram-molecule." 



As the curie is an r of radium emanation, the 

 adoption of a new name to denote the quantity r 

 will give two names for the same quantity of radium 

 emanation. The majority of those with whom the 

 subject has been discussed regarded this as undesir- 

 able. They consider it better to redefine the curie so 

 as to cover the entire field. 



I shall be glad if you will publish this letter so 

 that a further expression of opinion may be obtained. 

 A more detailed presentation of the subject will 

 shortly appear in the Journal of the Washington 

 Academy of Sciences. N. Ernest Dorsey. 



Bureau of Standards, July 30. 



Pisidium clessini in British Lochs. 



Dr. Annandale (Nature, August 18, p. 778) 

 assumes from Mr. B. B. Woodward's letter that this 

 species is a deep-water form, but this is not so. P. 

 clessini is abundant in some of the Welsh and Kerry 

 tarns, where Mr. Charles Oldham and I have collected 

 it in from i to 4 ft. of water. It is a form which 

 I have had under observation for some years past — 

 indeed, since I first collected it on Brandon Mountain 

 in Co. Kerry in 19 10. 



Not being able to identify it with any described 

 species of Pisidium, I have several times been on the 

 point of figuring it as new. At the last moment, 

 however, I have always been checked by the fact 

 that I was not satisfied that it was a good species. 

 This view, I mav say, is shared bv mv colleagues, 

 Mr. Charles Oldham and Mr. R. A. Phillips. We 

 are not satisfied that P. clessini is anything but a cold- 

 water (depauperate) form of the widespread P. 

 casertanum. This latter occurs abundantly also in 

 many mountain tarns, but is always — in our experi- 

 ence- — conspicuous by its absence in those in^ which 

 P. clessini occurs. 



Superficially, P. clessini is very distinct, and Dr. 

 Odhner is satisfied that its anatomical characters render 

 it necessary that we should regard it as a species ; 

 yet he has not been able to assure me that these \ 

 characters are not the result of starvation acting over ''. 

 a prolonged period on a number of generations. For 

 my own part I shall not be. satisfied in its standing 

 as a species until I can find it living in association 



