218 Lord Bayleigh [March 16, 



I must not dwell further upon isolated questions, however inter- 

 esting ; but will pass on at once to our main subject, which may be 

 divided into three distinct parts, relating namely to heat, especially 

 dark radiation, sound, and the behaviour of small particles, such as 

 compose dust, whether of living or dead matter. 



The earlier publications of Tyndall on the subject of heat are for 

 the most part embodied in his work entitled ' Heat as a Mode of 

 Motion.' This book has fascinated many readers. I could name 

 more than one now distinguished physicists who drew their first 

 scientific nutriment from it. At the time of its appearance the law 

 of the equivalence of heat and work was quite recently established by 

 the labours of Mayer and Joule, and had taken firm hold of the minds 

 of scientific men ; and a great part of Tyndall's book may be con- 

 sidered to be inspired by and founded upon this first law of thermo- 

 dynamics. At the time of publication of Joule's labours, however, 

 there seems to have been a considerable body of hostile opinion, 

 favourable to the now obsolete notion that heat is a distinct entity 

 called caloric. Looking back, it is a little difficult to find out who 

 were responsible for this reception of the theory of caloric. Perhaps 

 it was rather the popular writers of the time than the first scientific 

 authorities. A scientific worker, especially if he devotes himself to 

 original work, has not time to examine for himself all questions, 

 even those relating to his own department, but must take something 

 on trust from others whom he regards as authorities. One might say 

 that a knowledge of science, like a knowledge of law, consists in knowing 

 where to look for it. But even this kind of knowledge is not always 

 easy to obtain. It is only by experience that one can find out who 

 are most entitled to confidence. It is difficult now to understand the 

 hesitation that was shown in fully accepting the doctrine that heat is 

 a mode of motion, for all the great authorities, especially in England, 

 seem to have favoured it. Not to mention Newton and Cavendish, we 

 have Eumford making almost conclusive experiments in its support, 

 Davy accepting it, and Young, who was hardly ever wrong, speaking 

 of the antagonistic theory almost with contempt. On the Continent 

 perhaps, and especially among the French school of chemists and 

 physicists, caloric had more influential support. 



As has been said, a great part, though not the whole of Tyndall's 

 work was devoted to the new doctrine. Much relates to other 

 matters, such as radiant heat. Objection has been taken to this 

 phrase, not altogether without reason; for it may be said that 

 when heat it is not radiant, and while radiant it is not heat. The 

 term dark radiation, or dark radiance as Newcombe calls it, is prefer- 

 able, and was often used by Tyndall. If w T e analyse, as Newton did, 

 the components of light, we find that only certain parts are visible. 

 The invisible parts produce, however, as great, or greater, effects in 

 other ways than do the visible parts. The heating effect, for example, 

 is vastly greater in the invisible region than in the visible. One of 

 the experiments that Tyndall devised in order to illustrate this fact I 



