March 29, 1900] 



NATURE 



515 



drying up, and only a sufficient number to do the work being 

 retained on duty, the others going' to the reserve (Trans. 

 Roy. Soc. Edin. vol. xxxvii. Part ii. No. 20). It may be 

 also as well to remember that even supposing all the dust to be 

 precipitated out of the air, nature may be able to manufacture 

 fresh supplies {Trans. Roy. Soc. Edin. vol. xxxix. Part 

 i. No. 3). .If there are any gaseous impurities in the air, 

 such as ammonia, nitric acid, nitrous acid, peroxide of hydro- 

 gen, sulphurous acid, sulphuretted hydrogen, hydrochloric 

 acid or chlorine, the sunshine will convert them into nuclei, 

 which do not require a high degree of supersaturation to make 

 them active centres of condensation. 



There is another point in connection with condensation by 



means of ions which may be referred to here. It does not 



seem probable that ions could ever cause the formation of a 



cloud though they might give rise to rain. If the air becomes 



so supersaturated as to make them active centres of condensation, 



it will then be in an unstable condition. It will, in fact, be in 



an explosive state — explosive centripetally and not centrifugally 



1-; usual. Whenever any ion, in air in that condition, owing to 



ny advantage of its constitution, or difference of temperature, 



lessure, or saturation in the air surrounding it becomes active 



efore the others, it will grow extremely rapidly, and at once 



egin to fall ; and as it will fall through highly supersaturated 



ir, it will relieve the tension all along its path, and so grow 



ipidly, and soon become a rain-drop and fall to the earth. In 



Mr. Wilson's experiments cloudy condensation takes place with 



ns as nuclei, but in his apparatus the expansion is nearly in- 



antaneous, and no particular ion has time to take advantage 



V er the others, and many are thus formed at the same instant. 



There is still another point which requires consideration. What 



would the effect be in a rising column of air if it could not call 



on the reserve heat, latent in its vapour, till it had risen to a 



higher elevation than is necessary when dust is present. 



So far as our knowledge goes, it can hardly be said there is 

 >uch a thing as dust-free air in our atmosphere, and the cases 

 in which low numbers have been observed are so extremely 

 care that they can hardly have any bearing on a phenomena of 

 such widespread existence as atmospheric electricity, even 

 'hough we suppose those few particles to be afterwards got rid 

 f. We cannot suppose the positive ions will always retpain in 

 le atmosphere, because if the conditions are ever such as to 

 cause the fall of the negative ions, the positive ones will also 

 Afterwards fall with only a slightly greater increase in super- 

 saturation. 



From what has been stated above, I think we must defer ex- 

 pressing an opinion on the value of this theory of the source of 

 atmospheric electricity, and wait till some stronger evidence is 

 produced that the air in our atmosphere is ever absolutely dust- 

 free, so as to permit of the supersaturation becoming great 

 enough to cause a separation of the negative and positive ions. 

 Ardenlea, Falkirk, March 17. John Aitken. 



Escape of Gases from Planetary Atmospheres. 



In the " Astronomical Column" of last week's Naiure, on 

 p. 501, you give an abstract of a paper by Mr. S. R. Cook, of 

 the University of Nebraska, and quote the passage in which he 

 points out that the present writer, when investigating the escape 

 of gases from atmospheres, does not base his argument upon 

 -" the determination by the kinetic theory of the relative number 

 of molecules which would have a velocity sufficient to enable 

 them to escape from the earth or planet." 



This is so : and the reason is that no such determination 

 existed until that arrived at in the paper criticised by Mr. Cook, 

 where data drawn from outside the kinetic theory are employed 

 to supplement what the kinetic theory teaches. These auxiliary 

 data are (l)that the moon has not retained an atmosphere; 

 and (2) that the earth and Venus do retain the vapour of water 

 in their atmospheres. 



Mr. Cook supposes that Maxwell's law for the distribution of 

 the speeds of the molecules when a gas exists under normal 

 conditions may legitimately be employed to obtain the rate of 

 the escape from an atmosphere. But in this he overlooks (i) 

 the fact that the molecules that escape are emitted exclusively 

 from that outermost layer of the atmosphere, throughout which 

 the molecules are within striking distance of the void space 

 beyond ; and (2) the important circumstance that the mole- 

 cules exist within this altogether special layer under conditions 

 entirely remote from those which are assumed by Maxwell in the 

 XO 1587. VOL. 61] 



proof 1 of his law ; so that Maxwell's law fails us just where we 

 want its help, viz. in that part of the aVmosphere from which the 

 entire of the escape of molecules takes place. 



This upper stratum of the atmosphere, which is probably some 

 miles in depth, is limited on its inner side by a deeper-seated 

 stratum of air, and on the outer side by a stratum of virtually 

 empty space, that is, by a space tenanted only by molecules which 

 seldom or never meet with encounters while within that space, 

 and are therefore to be regarded as temporarily or permanently 

 beyond the atmosphere. As to the stratum which is above spoken 

 of as the outermost of the atmosphere, molecular encounters 

 take place in it ; and whenever an unusual speed is generated in 

 any of the molecules which occupy it (as happens frequently to 

 every molecule of a gas) these molecules have an opportunity of 

 placing themselves beyond the reach of those subsequent en- 

 counters which, in gas under normal conditions, are what tone 

 down the frequently recurring irregularities, and bring about 

 an approximate conformity with Maxwell's law within a suffi- 

 cient volume of gas of uniform density, and surrounded by gas 

 of the same density. 



Nevertheless, the numerical results obtained by Mr. Cook, 

 though arrived at by a faulty process, are not useless. They 

 have a certain value, inasmuch as it can be proved that the 

 actual escape of gases from an atmosphere is more rapid than it 

 would be if Maxwell's law governed that rate. Accordingly, 

 Mr. Cook's numbers furnish a computed rate which we know that 

 the actual rate must exceed, and may largely exceed. This in itself 

 is valuable information ; and would be important information, if 

 we had no better way of investigating the problem. 



Like Mr. Cook, the present writer, when he first entered on 

 the investigation of the escape of gases from atmospheres in 

 1867 or 1868, hoped that Maxwell's law for the distribution of 

 the speeds of the molecules under normal conditions would 

 render aid ; and it was only when he found that to conduct 

 the inquiry in this way could not furnish correct results, that 

 he cast about for some other way of approaching the problem, 

 and finally adopted that which is developed in his memoir. •' 

 G. Johnstone Stoney. 



8 Upper Hornsey Rise, N., March 25. 



State of Practical Instruction in Physics, 



With the view of obtaining comparative statistics of the 

 organisation of practical instruction in physics, I have recently 

 sent a list of questions, with table of different manipulations, to 

 the directors of all the physical laboratories with the addresses 

 of which I am acquainted. 



Through the medium of your esteemed journal, may I ask the 

 directors of the laboratories, who, by any reason whatever, have 

 not received this list, kindly to inform me, so that I may at once 

 forward the same. I should like to make the same appeal to 

 the directors of the mechanical, electrical engineering, electro- 

 chemical, physico-chemical, &c., laboratories where part of the 

 practical work is of a purely physical character. 



Boris Weinberg. 



University of Odessa, Russia, March 17. 



Indian Corn. 

 I have just found in Nakamura's " Kimmo Dzui," first 

 edition, 1666, Book xvi. fol. -jb, a Japanese wood-cut of Indian 

 corn, with its Japanese and Chinese names as I gave in my 

 previous letter (p. 392, ante). This figure proves that, though 

 Kaempfer does not mention the plant in his " History of Japan," 

 1727, yet, through his seeing to it, he must have recognised as 

 a fact the introduction of maize to Japan before the time of his 

 sojourn in it ; for most illustrations of the biological objects in 

 his noted " History" (vol. i. tab. ix.-xiv.) are actually found 

 to have been reproduced from the above mentioned, once very 

 popular, Japanese cyclopiedia (Books xii.-xv.). 



KUMAGUSU MiNAKATA. 



I Crescent Place, South Kensington, S.W., March 9. 



The Bacteriology of the Soil. 

 I should be much obliged if, through the columns of Nature, 

 you would give me the names of the best books in English or 

 German which deal with soil (agricultural) bacteria. 



Thos, T. Watson. 

 Rosely Cottage, Collier Street, Carnoustie, March 26. 



1 See Pkii. Mag. for January i860, p. 22 ; or vol, i. of his " Collected 

 Papers," p. 380. 



- See Scientific Transactions of the Royal Dublin Society, vol. vi. Part 

 13 ; or A strophysical Journal iot January 1898. 



