462 



NATURE 



{April 13, 1876 



men — who wished to obtain in this way a notion of the 

 chief advances made in Natural Philosophy since their 

 student days. The demand, therefore, which these lec- 

 tures are intended to meet, is that of men who, though 

 they have received a liberal education, in which the ele- 

 ment of science has not been neglected, are too deeply 

 engaged in their professional work to keep themselves 

 abreast of contemporary science by regular study, but 

 who are yet able to avoid falling behind by occasionally 

 availing themselves of an hour with a scientific friend. 



In lectures of this kind, therefore, we are not to look 

 for the elaborate exposition and reiterated inculcation by 

 which the facts and methods of science are impressed 

 upon the minds of beginners. Still less are we to expect 

 the forcible language and striking illustrations by which 

 those who are past hope of being even beginners may be 

 prevented from becoming conscious of intellectual ex- 

 haustion before the hour has elapsed. We are rather to 

 listen to one who has climbed high on the hard and slip- 

 pery peaks of science as he points out the grand features 

 of the prospect to those who stand on a lower level but 

 yet on the same solid foundation with himself. 



In his own words he has " to point out a few of the 

 principal peaks which we have to ascend, and of the more 

 formidable abysses which we have to avoid." 



As safety must come before success, it may be well to 

 observe that the most formidable of all these abysses is 

 that of a priori physics. The study of this branch of 

 science as it is to be found in the works of Hegel, Her- 

 bart, and others, seems to furnish an unfailing source 

 of recreation to those who are engaged in the less 

 amusing researches of experimental physics. In our 

 modern examinations those candidates who try to 

 conceal their ignorance by sending up what appears 

 to them to be the most plausible answer to a ques- 

 tion, often help to relieve by their felicitous absurdities 

 the tedious labours of the examiner. We have only to 

 imagine that instead of the weary examiner we have the 

 vigorous man of science, and instead of the timorous 

 candidate, some great philosopher before whose inner 

 vision the whole world of being and not-being, in its 

 apparent contradiction and fundamental indifference, lies 

 open ; and we shall then have some faint idea of the 

 mode in which the writings of these philosophers may be 

 destined to contribute to the merriment, if not to the 

 happiness of the coming race. 



We are glad to find, however, that in spite of the con- 

 tempt which Prof. Tait pours upon the a priori physics 

 of non-experimental philosophers, he admits that there 

 is a true science of metaphysics which discusses the 

 fundamental ideas of all science and knowledge, not by 

 shutting out all the facts of experience, but by calling in 

 all the evidence obtainable from the whole circle of the 

 science. 



In fact one of the greatest benefits which the advance 

 of science has conferred on the world at large is that 

 words and phrases have been gradually introduced into 

 ordinary language which are consistent with true scien- 

 tific ideas, and that these have displaced words and 

 phrases which implied false ideas about nature, so that 

 each generation, as it learns its mother tongue, finds it 

 better adapted to express what really exists, and less 

 suggestive of what is not. 



We have only to read the expositions of science in the 

 seventeenth century to see that they are addressed to 

 students whose minds were imbued with prejudices and 

 superstitions which are now known only to archaeologists. 

 Those who have the good fortune to be born in these 

 latter times can hardly realise the reasons why certain 

 natural phenomena rather than others were dignified or 

 stigmatised with the name of paradoxes. 



The man of science, if he confines himself to writing 

 scientific books, can influence only the professed students 

 of science, but if he can find an audience among men of 

 business and men of action, who desire to keep up their 

 scientific knowledge, he will at the same time help them 

 to keep up a scientific habit of thought and expression. 

 In this way a course of lectures like those of Prof. Tait 

 may do something towards infusing a scientific spirit into 

 the affairs and phraseology of business life, and since it is 

 of the essence of science to speak of things as they are, the 

 business phrases which satisfy this condition will gradu- 

 ally but surely displace those which describe things as 

 they are not. 



The subjects discussed in • these lectures are the con- 

 servation, transformation, and dissipation of energy, 

 spectrum analysis, the conduction of heat, and the 

 structure of matter. 



The experiments by which the lectures are illustrated 

 are many of them new, and all of them well described. 

 We may take the following as an example of Prof. Tait's 

 method of illustrating the connexion of radiation with 

 absorption : — 



" I can show to a few at a time, but not in a marked way 

 at a distance, the same phenomenon [as in Stewart's 

 experiment with pottery], by taking a piece of platinum 

 foil and writing letters upon it with ink. When it is once 

 heated there is a deposit, on the surface of the otherwise 

 polished platinum foil, of oxide of iron which tarnishes 

 the surface and makes it absorb considerably more light 

 than a polished reflecting surface will do. We should 

 expect, then, when this is heated (as I now heat it in a 

 powerful but very slightly luminous flame), and becomes 

 in turn the source of light, to see bright letters on a dark 

 ground. The difference of brightness is not so marked in 

 this case as in the last, but still those who are nearest to 

 me will see the phenomenon distinctly enough. 



" But you will see another phenomenon still more 

 startling on looking at the back of the heated foil instead 

 of the front of it. You see faint traces of bright letters on the 

 dark ground when I turn the inked, side to you, but when 

 I turn the other side you see dark letters on a bright 

 ground. Now, the reason why on the one side we have bright 

 letters on a dark ground, while the other side of the same 

 piece of metal shows dark letters on a white ground, is still 

 more confirmatory of the result of Balfour Stewart's experi- 

 ment, which I have just stated, because these letters appear 

 dark while at present cold, because they are absorbing 

 more than the rest of the polished surface. They appear 

 brighter than the polished surface when heated, because 

 they radiate more ; but just because they radiate more 

 they must become colder — must be kept permanently 

 colder than the rest of the foil, and therefore the parts at 

 the back of the foil, behind those which are radiating 

 most, remain permanently colder. This is made evident 

 when we look at the side which is without any difference 

 of surface, as we then see, by the relative amounts^ of 

 brightness, a marked distinction between the parts which 

 are hotter and those which are colder. This is a still 

 more complete proof of Stewart's proposition." 



We may also notice Prof. Tait's exposition of his views 



