NA TURE 



433 



SIR WILLIAM THOMSON'S POPULAR 

 LECTURES. 



Popular Lectures afid Addresses. By Sir William 

 Thomson, LL.D., F.R.S., F.R.S.E., &c. In Three 

 Volumes. Vol. I. Constitution of Matter. With Illus- 

 trations. (" Nature Series.") Pp. xi. + 460. (London : 

 Macmillan and Co., 1889) 



'^jPO review a book by the leader of British physical 

 -JL science in the ordinary sense of reviewing would 

 be absurd. To attempt an estimate of merit and demerit 

 or to offer a superficial criticism might easily become an 

 impertinence. 



The object of a review in such a case as this is mainly 

 to give persons who have not concentrated their attention 

 on physics some idea of the nature of the book, so as to 

 enable them to form a judgment how far it is suitable 

 and accessible to them. 



For, inasmuch as the greater part of what is published 

 by the author of this book is stiff reading for trained 

 physicists and mathematicians, and inasmuch also as the 

 subjects of which he treats even in popular lectures are 

 usually extremely abstruse, and such as require, if they 

 are to be accurately stated at all, a very carefully-selected 

 form of words and a rather involved construction of 

 sentences, the idea may easily grow that anything by 

 Sir William Thomson is mainly unintelligible. And 

 unintelligible it probably is to the general public in 

 their after-dinner arm-chairs. Unintelligible it quite 

 possibly was to a large percentage of the audience in 

 their after-dinner seats at the Royal Institution, though 

 the personality of the man and the magnetism of his 

 enthusiasm could hardly fail to enchain the attention of 

 the most cynical or casual hearer. 



In the printed book this personal charm is fainter ; it 

 is not absent — to those who have ever heard him the 

 manner in which the illustrations are brought forward, 

 the very tone of voice with which the sentences were 

 delivered, are continually suggesting themselves — but it 

 is fainter ; and it becomes a question how far these 

 lectures, which are undoubtedly scientific, are really 

 popular, i.e. are really adapted to intelligent persons 

 interested generally in the subject but who make no 

 claim to be specialists in it. To answer this question I 

 will run through the contents in such order as may be 

 convenient. 



About the middle of the book there is a quite popular 

 and easy essay on the sense-organs of man, including 

 that most important and fundamental sense— the sense 

 of muscular exertion — without which it is doubtful if we 

 should be conscious of an external world at all, insisting 

 on a distinct sense of heat, and lumping together taste 

 and smell ; an essay in which certain much-needed 

 clarifying statements are made to counteract some con- 

 fusions introduced by more than one very popular book, 

 as, for instance, the familiar difficulty about the relation- 

 ship between light and radiant heat. 



Next comes a lecture on the wave theory of light, 

 delivered in America, giving to anyone who has attended 

 Vol, XL.— No. 1036. 



a course of lectures or read some popular treatise on light 

 a very good general notion of what is meant by the elastic- 

 solid theory of the ether, and of the way in which the 

 difficulties introduced by supposing light to consist of 

 ordinary mechanical transverse vibrations of an elastic 

 medium have to be met. 



Then follows a perfectly beautiful series of discourses 

 or articles on the age of the sun's heat, which, looked at 

 from the point of view of the general reader, perhaps 

 form the gem of the whole. 



The cool collected way in which a possible and more 

 or less probable way of forming our sun is gone into, 

 with every detail clear-cut and closely reasoned out, forms 

 a study than which nothing more instructive, more sugges- 

 tive, and more wildly interesting is likely to be accessible 

 with equal ease to the imaginative reader. 



A sustained power of attention, a period free from 

 interruption, and a power of forming vivid conceptions, 

 are all that is needed for a comparatively uninstructed 

 reader to receive some of the most splendid cosmical 

 speculations of our time. He may not know exactly why 

 when the two earth-like bodies start to rush together it is 

 stated that they will meet in six months, or that the col- 

 lision will last half an hour, and perhaps he may find some 

 difficulty in picturing the equatorial zone or disk and the 

 axial rod between which forms the mass will subsequently 

 oscillate till it settles down into a globular and white-hot 

 sun ; but he may rest assured that none of these state- 

 ments nor any such numerical statements met with in 

 this book are random ones ; they are all the result of 

 exact mechanical knowledge and arithmetic, and whether 

 they be precisely true or not, they are, at all events, 

 righter than anything else he is likely to come across. 



Irrespective of that on which stress is laid in the title, 

 viz. the age of the sun's heat, we have in these essays a 

 popular and very clear exposition of the solution of that 

 long-standing puzzle — the means by which solar heat is 

 maintained. 



With lumps of matter of ordinary size {i.e. not incom- 

 parably greater or less than the human body) gravitation 

 is a force altogether insignificant in comparison with 

 chemical affinity, and accordingly while the combustion 

 of a lump of coal transforms great quantities of energy, 

 the force of gravitative attraction between two such lumps 

 or between one lump and the oxygen it can combine 

 with is so minute as to require a Cavendish and a Boys 

 to demonstrate its existence to an audience. But with 

 lumps of matter of sizes such as are found in the depths 

 of space the case is quite otherwise. Between them 

 gravitative attraction is furiously greater than any known 

 kind of chemical affinity ; and the work such masses can 

 do in falling together, nay even the work one lump can 

 do in slowly contracting upon itself, is sufficient to main- 

 tain radiation at the sun's prodigious rate. 



Take a large enough mass of gas {i.e. of detached 

 atoms), let its parts gravitate together continually, and 

 you have a sun — a sun, moreover, obeying simple 

 mechanical laws, and with a life-period, in its molten 

 and therefore uncrusted and radiative state, of roughly 

 calculable length. 



It may be worth while parenthetically to remark that, 

 whereas the chemical (or electrical) attraction between 

 two atoms at any distance exceeds their gravitative 



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