June 14, 1900] 



NATURE 



49 



equipped laboratory is one of the most important and 

 necessary parts of a cyanide plant. The control, testing, 

 and analysis of solutions is treated in a fuller manner 

 than is usual with books of this class, and of the three 

 methods given we prefer the silver nitrate test. The 

 tables for the assay of cyanide solutions are a useful 

 addition to this chapter. The appliances for cyanide 

 extraction are briefly described, and although accom- 

 panied by several good scale drawings, certain details 

 are omitted which might have been profitably included. 

 The synopsis of the process for the actual extraction by 

 potassium cyanide is well written, and the conditions for 

 successful treatment, such as strength of cyanide solu- 

 tion, &c., are stated as clearly as one could wish. 

 Chapter vii. deals with the applications of the processes 

 at different works. Leaching and precipitation are suc- 

 cinctly dealt with in Chapters viii. and ix. These are 

 followed by a short description of the Siemens- Halske 

 electrical process, which not only deposits the gold, but 

 gives rise to the production of a number of valuable 

 commercial bye-products, such as lead, copper, litharge 

 and paint. For all those who wish to obtain a sound 

 knowledge of the cyanide process, as conducted at the 

 present time, we heartily commend Park's handbook. 



OUR BOOK SHELF. 



The Cause and Prevention of Decay in Teeth. By J. Sim 



Wallace, M.D., B.Sc, L.D.S. Pp. loi. (London : 



J. and A. Churchill, 1900.) 

 This is a reproduction in book form of a series of 

 articles published in the Journal of the British Dental 

 Association. 



The subject has been dealt with in the light of the 

 now universally accepted chemico-parasitic theory of 

 dental caries, but the author treats less of exciting or 

 immediate causes than of those remote and predisposing. 

 He attributes the great and increasing prevalence of 

 dental caries among civilised nations to the elimination 

 of the coarser and more fibrous parts of foodstuffs from 

 the diet, and points out that this may act in two ways. 

 Firstly, owing to the absence of mechanically detergent 

 constituents of food, more of the fermentable, acid- 

 producing and germ-sustaining parts of the latter re- 

 main in contact with the teeth for some time after meals. 

 Secondly, that the tongue, being less actively employed 

 during the act of chewing and swallowing, fails to attain 

 its full size and exercise its normal important function 

 in modelling the dental arches, so that irregularities 

 arising from crowding and malposition of the teeth serve 

 to intensify their predisposition to caries. 



The subject is, on the whole, efificiently dealt with, and 

 the book may be recommended to the medical practitioner 

 or intelligent layman. 



It is a pity, however, that the author lays such per- 

 sistent stress upon what he considers the daring 

 heterodoxy of his opinions, as these are at most modi- 

 fications of those currently accepted. It is somewhat 

 irritating, too, to find set forth for the instruction of the 

 dentist, and with an air of great originality (as on p. 94), 

 certain points in the operative treatment of caries which 

 are among the very first impressed upon all students in 

 schools of dental surgery. 



Surely, too, the accusation of ignorance of the causes 

 of the diseases he attempts to combat, and empiricism in 

 practice, are undeserved by the educated dental surgeon 

 of to-day. HAROLD AUSTEN. 



NO. 1598, VOL. 62] 



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 correspond with the writers of, rejected 

 manuscripts intended for this or any other part 0/ Nature. 

 No notice is taken of anonymous communications.^ 



Atmospheric Electricity. 



In a letter on this subject in Nature of March 29, Mr. 

 Aitken criticises the theory which attributes the prevalence of 

 positive electrification in the atmosphere to the superiority in 

 efficiency as nuclei for the condensation of water vapour, of the 

 negative ions over the positive. 



That any difference in the degree of supersaturation necessary 

 to make water condense on positively and on negatively charged 

 ions would result under suitable conditions in the production of 

 an electric field was pointed out by Prof. J. J. Thomson [Phil. 

 Mag. vol. xlvi. p. 533), and it was suggested by him that this 

 might be a source of atmospheric electricity. Experiments 

 made by the present writer proved that there is such a difference, 

 and that water vapour condenses much more readily on negative 

 than on positive ions ; while Elster and Geitel (and independ- 

 ently, Lenard) have recently brought forward evidence based 

 on their own experiments and those of Liuss, tending to show 

 the existence of free ions in the atmosphere. 



There remains the question whether the necessary degree of 

 supersaturation can ever occur in the atmosphere. Mr. Aitken 

 contends that there is no such thing as dust-free air in the atmo- 

 sphere, and that therefore any considerable degree of super- 

 saturation is impossible. 



Air practically dust-free does, however, seem to have been 

 met with on Ben Nevis, accompanied by something very like 

 supersaturation ( Rankin, yt;M;-«. Scot. Met. Soc. vol. ix. p. 131). 

 In Mr. Aitken's own papers, too, records of small numbers of 

 dust particles (sometimes considerably less than looperc.c.) 

 are not rare ; and the lowest values are met with just under the 

 conditions where their occurrence is of most significance. For 

 " most of the low numbers in the tables were observed during 

 rainy weather, and the very low ones in misty rain, when the 

 clouds were at or near the surface of the earth" (Aitken, 

 Edin. Trans, xxxvii. p. 664). Again, the purest air met with 

 by Mr. Aitken was that blowing from oft" the Atlantic Ocean, 

 the mean number of dust particles in a series of 258 observa- 

 tions extending ever nearly five years amounting to 338perc.c. ; 

 on one occasion the number was as low as 16 per c.c. (Edin. 

 Trans, xxxvii. p. 666). Air coming from such a region can 

 hardly be considered as abnormal. Moreover, such observations 

 are necessarily made in air within a few feet of the ground ; at 

 a greater height it is likely to be less contaminated. 



Consider a mass of air occupying i c.c. and saturated with 

 water- vapour at 10° C, and let it expand till, say, 3 x 10"® gram. 

 (less than one-third of the total water) has condensed to form 

 100 drops. Let us suppose the drops to be equal in size and let 

 us calculate the volume and thence the radius of each drop, and 

 from this obtain the rate at which they will fall relatively to the 



air (assuming the velocity =-g ~, the viscosity /u being taken as 



9 M 

 I "8 X io~*). We obtain for the radius of each drop the value 

 1-9x10"* centim., and for the rale of fall through the air, 

 z; = 44 cms. per second. 



In a rising current of moisture-laden air containing 100 dust 

 particles per c.c. there is thus no difficulty in seeing how the 

 drops as they ascend may grow large enough to lag behind the 

 air at the rate of 4-4 cms. per second ( = 160 metres per hour) ; 

 while the greater part of the moisture in the surrounding 

 air is still retained as vapour. If then the upper surface of the 

 cloud is carried to such a height that the drops reach the size 

 r=r9x io~^ cm., it will there be lagging behind the rising 

 air at the rate named, and a dust- free layer must exist immediately 

 above it, increasing in vertical thickness at the rate of something 

 like 180 metres per hour. Even if 1000 drops were formed in 

 each c.c. of the cloud, the rate of growth of the dust-free layer 

 would, as a similar calculation shows, when the same quantity of 

 water had separated, amount to 34 metres per hour. 



A difficulty raised by Mr. Aitken in connection with the 

 removal of dust particles by condensation of water upon them 

 is this : " When a cloud forms in ordinary impure air, only a 

 small proportion of the dust particles become active centres of 



