May 20, 1922] 



NA TURE 



645 



Ministry of Munitions and Department of Scientific 

 and Industrial Research. Technical Records of 

 Explosives Supply, igrj-igiS. No. 8 : Solvent 

 Recovery. Pp. iv + 22. (London: H.M. Stationery 

 Office, 192 1.) 2,s. net. 



In the manufacture of cordite, which is the propellant 

 used in practically all arms in warfare, a mixture of 

 nitrocellulose and nitroglycerin is incorporated with a 

 ■' solvent," consisting of ether and alcohol, and the 

 doughy mass is extruded through dies to form the 

 cordite strands. These are dried on trays in closed 

 recovery stoves, where the solvent is evaporated in a 

 current of warm air until only a small amount of 

 volatile matter remains, which is finally expelled in 

 drying stoves. The solvent-laden air may be treated 

 in absorbers for the recovery of the solvents. The 

 present report deals with the use of sulphuric acid, 

 water, and cresol as absorbents, the last being found 

 most satisfactory. The air and absorbent were brought 

 together in a Whessoe scrubber, such as is used in 

 gas works, and the solvent then expelled by distilla- 

 tion. Calculations dealing with the operation of the 

 plant are given. 



A Manual of Selected Biochemical Methods as Applied 

 to Urine, Blood, and Gastric Analysis. By Prof. 

 F. P. Underhill". Pp. xiv + 232. (New York: 

 J. Wiley and Sons, Inc. ; London : Chapman and 

 Hall, Ltd., 192 1.) 175. 6d. net. 



A COLLECTED account of the various ingenious methods 

 devised by American workers in the field of urine, 

 blood, and gastric analysis will be found in this useful 

 laboratory manual. Although doubtless the methods 

 are adequate for the purposes described, it is some- 

 what surprising to find no reference to the Barcroft 

 apparatus for determining oxygen capacity, nor to the 

 almost indispensable comparator of Cole or Walpole 

 for use with indicators in coloured solutions. Mett's 

 tubes require more cautious criticism in quantitative 

 work than is suggested by- the author. These are 

 perhaps minor blemishes, and, apart from them, the 

 book can be highly recommended. It is to be feared, 

 however, that the price will militate somewhat against 

 a large sale in this country. 



The Commercial Apple Industry of North America. By 

 J. C. Folger and S. M. Thomson. (The Rural Science 

 Series.) Pp. xxii-f- 466 + xxiv Plates. (New York : 

 The Macmillan Company ; London : Macmillan and 

 Co., Ltd., 1921.) 185. net. 



A FULL account of the growing of apples on a com- 

 mercial scale in North America is given in this work, 

 and much information that could be obtained only 

 with difficulty elsewhere is embodied in the text. It 

 would prove useful to any English grower or student 

 of horticulture who wished to obtain information as to 

 the way in which this important industry is carried on. 

 The authors state in their introduction that they have 

 visited practically every important apple-growing 

 county in the United States, first in connection with an 

 investigation into the cost of production, and later in 

 connection with attempts to organise a system for 

 estimating the apple crop of the United States. 



NO. 2742, VOL. 109] 



Letters to the Editor. 



[The EdUor 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 mattuscripis intended for 

 this or any other part of NATURE. No notice is 

 taken of anonymous communications.^ 



On Immediate Solutions of Some Dynamical 

 Problems. 



As a branch of science advances and its principles 

 become more familiar to the mind of the investigator 

 many things which before appeared involved and 

 mysterious become simple and clear, and it is possible 

 to find proofs of theorems so obvious and brief as 

 to merit the name intuitive in a very real sense, 

 though not that in which the term is frequently 

 applied. For to say that a theorem or principle is 

 intuitively perceived is often tantamount to saying 

 that it is not perceived at all. By an intuitive proof 

 of a proposition I mean a proof which is natural 

 and direct, and it may be almost instantaneous in 

 that the restatement of some element of the proof 

 transforms the whole so that the proposition is at 

 once recognised to be true. But the proof must be 

 complete and rigid to be valid, and completeness 

 and rigidity are qualities which have come to be 

 almost denied by calling a proof " intuitive." 



I have amused myself from time to time with 

 endeavouring to devise what I venture to think are 

 properly called immediate proofs of dynamical pro- 

 positions, and some of these, with historical notes 

 here and there, may be of interest to readers of 

 Nature. Many of the ideas of attractions have 

 become so familiar, not to students generally by 

 any means, but to those who have pondered over 

 the connection betv/een the theory of gravitational 

 attraction and the mathematical theory of electro- 

 statics for example, that the subject has acquired a 

 ven,' special interest and fascination to the minds 

 of such workers. Accordingly I give here some 

 propositions in attractions. 



It is undoubtedly the case that Newton delayed 

 the publication of the discovery of universal gravita- 

 tion until he had discovered a proof which satisfied 

 him that a uniform spherical shell attracts an external 

 particle, as it would if the whole mass of the shell 

 were comprised in a particle situated at the centre. 

 For if this proposition were established, the earth, 

 which there was reason to believe was a nearly 

 spherical body with a distribution of density ap- 

 proximately symmetrical about the centre, would 

 attract external matter as if its whole mass were 

 collected at the centre, and this therefore was the point 

 from which distances were to be measured in the 

 numerical comparison of gravitational forces ; for 

 example, the comparison of the two unital attractions 

 of the earth, that on a^particle at the surface and that 

 on the moon. 



The proposition given by Gauss that the surface 

 integral of normal force taken over a closed surface 

 drawn in the field is equal to ^^irk times the whole 

 quantity of the attracting matter which is contained 

 within the closed surface, is capable of many appUca- 

 tions. This proposition may be more precisely stated 

 as follows : Let dS be an element of area of the 

 surface and N be the component of the field intensity 

 at right angles to the surface (taken positive when 

 acting outwards) . Then the integral 



/NVS, 

 taken over the closed surface, is called the surface 



Y 2 



