548 



NATURE 



[October 8, 1903 



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.] 



Expansion Curves. 

 Every man who has studied steam or gas or oil engines 

 knows that if there is one construction more important than 

 another it is to draw a curve representing the law 



/)7>»= constant 



through any given point. Here is an exceedingly simple, 

 ingenious method of doing this which I have just found 

 in a pamphlet by Mr. E. J. Stoddard, of Detroit. Let A 

 be the given point so that AB represents a given volume. 



and AG a given pressure. Set off any convenient angle, 

 VOJ=o say. Compute an angle ^ such that 



I + tan )3 = (i + tan a)", 



and set off BOC = 3. Produce AG to J. Now make OBC = 

 JHO=45°, and project from C and H to find E a point in 

 the curve. The proof is obvious. 



It is evident that OC may be drawn to the right of OB, 

 and OJ above OV, to save paper if necessary. 



J. Perry. 



Royal College of Science, S.W., September 23. 



Botany in Boys' Schools. 



Prof. W. W. Watts said in his address to the Geology 

 Section of the British Association, " there is no science in 

 which materials for elementary teaching are so common, so 

 cheap, and everywhere so accessible." 



In the light of this statement I sought material for the 

 teaching of another science — botany — in a north London 

 playground last week. 



The Angiosperms were represented by thirteen natural 

 orders. With a single representative each of the algae and 

 fungi, thirty-eight species in all were found growing in 

 or on a soil which is almost entirely ballast ! 



It seems a pity that botany should be so rarely taught 

 in boys' schools when a single playground yields materials 

 " so cheap and so accessible." H. J. Glover. 



Stationers' School, Hornsey, N., September 23. 



NO. 17 7 1, VOL. 68] 



Radium and the Cosmical Time Scale. 



Certain letters have appeared in Nature upon the bear- 

 ing of the properties of radium upon the cosmical time 

 scale. These letters are based on the assumption that 

 radium, or some equally active body, exists in the sun and 

 contributes materially to the output of solar energy. If 

 this assumption were true, we ought, I think, to be ablr 

 to detect the rays peculiar to radio-active bodies on tlv 

 surface of the earth — they should bear some proportion ti- 

 the great stream of light and heat waves which reaches us. 



Now a solution of iodoform in chloroform is very sensitive 

 to the )3 and 7 rays. A purple coloration is produced by 

 the rays from 5mg. of radium bromide even after filter- 

 ing through icm. of lead. On the other hand, I find that 

 direct sunlight (if heating be obviated) has no action when 

 the thinnest opaque screen is interposed even after many 

 days. Some of my solutions are now nearly two months old, 

 and they have been exposed in light-tight cardboard boxes 

 to such sunshine as has reached us during that period. 

 They are quite unchanged. 



It is, of course, possible that the stream of rays needs 

 to be above a certain critical density in order to decompose 

 the iodoform, but in any case my experiments prove that 

 the and y rays reach us at most only in faint quantities 

 from the sun. W. B. Hardy. 



Gonville and Caius College, Cambridge. 



Loss of Weight of Musk by Volatilisation. 



I SHOULD like to direct the attention of your correspon- 

 dent " S. W." (p. 496) to N. Cimento for May, 1902 (or 

 abstract 1986, Science Abstracts, 1902), in which 

 E. Salvioni says that he has shown the loss of weight of 

 musk by volatilisation. 



The measurements were made by a special form of 

 balance. p. R. Sexton. 



Park Lodge, Kingston-on-Thames, September 5. 



CONDENSATION NUCLEIC 



T N a previous paper under the not very appropriate 

 -^ title " Experiments with Ionised Air," Prof. Barus 

 has described observations, made by means of his 

 modified steam-jet methods, upon the nuclei found in 

 air which has passed over phosphorus, together with 

 measurements of the electrical lealcage through air 

 thus treated. The first chapter of the present volume 

 is taken up with a continuation of the work by the 

 methods there described. 



There is no reason to expect the properties of air 

 v^-hich has been exposed to phosphorus to be character- 

 istic of ionised air generally; the recent experiments 

 of Harms, and of Elster and Geitel, have, it is true, 

 shown that ions are probably present, but the con- 

 ditions are much more complicated than in cases of 

 simple ionisation, such as that due to X-rays, owing 

 to the presence of the products of the oxidation of the 

 phosphorus. It is probably to the presence of the pro- 

 ducts of the oxidation of phosphorus vapour, as was 

 pointed out In 1866 by Schmid, that the formation of 

 the phosphorus cloud is due. The cloud nuclei are 

 not free ions; in the " experiments with ionised air " 

 it was found that the number of nuclei was un- 

 diminished by even a strong electric field ; additional 

 evidence is brought forward In the first chapter of the 

 present paper, where experiments are described show- 

 ing different temperatures for the maxima of nucle- 

 ation and of ionisation. But such evidence was not 

 required to show that these nuclei are not ordinary 

 free ions, for in dust-free air ionised by X-rays or the 

 rays from radio-active substances (in all cases, in- 

 deed, in which the ions have the normal velocity under 



1 " The Structure of the Nucleus, a continuation of _' Experiments with 

 Ionised Air.' " By Carl Barus. (Smithsonian Contributions to Knowledge, 

 Hodgkins Fund, 1903.) 



