1883. J Professor Tijndall on Thoughts on Badiation. 25 



Q 



WEEKLY EVENING MEETING, 



Friday, March 16, 1883. 



William Bowman, Esq. LL.D. F.E.S. Honorary Secretary and 



Vice-President, in the Chair. 



Professor Tyndall, D.C.L. F.E.S. M.E.L 



Thoughts on Badiation, Theoretical and Practical. \ 



Scientific discoveries are not distributed uniformly in time. They 

 appear rather in periodic groups. Thus, in the tvTo first years of this 

 century, among other gifts presented by men of science to the world, 

 we have the Voltaic pile; the principle of Interference, which is 

 the basis of the undulatory theory of light ; and the discovery by 

 William Herschel of the dark rays of the sun. 



Directly or indirectly, this latter discovery heralded a period of 

 active research on the subject of radiation. Leslie's celebrated work 

 on the Nature of Heat was published in 1804, but he informs us, in 

 the preface, that the leading facts which gave rise to the publication 

 presented themselves in the spring of 1801. An interesting but not 

 uncommon psychological experience is glanced at in this preface. 

 The inconvenience of what we call ecstacy, or exaltation, is that it is 

 usually attended by undesirable compensations. Its action resembles 

 that of a tidal river, sometimes advancing and filling the shores of 

 life, but afterwards retreating and leaving unlovely banks behind. 

 Leslie, when he began his work, describes himself as " transported at 

 the prospect of a new world emerging to view." But further on the 

 note changes, and before the preface ends he warns the reader that he 

 may expect variety of tone, and perhaps defect of unity in his disqui- 

 sition. The execution of the work, he says, proceeded with extreme 

 tardiness ; and as the charm of novelty wore ofi", he began to look 

 upon his production with a coolness not usual in authors. 



The ebb of the tide, however, was but transient ; and to Leslie's 

 ardour, industry, and experimental skill, we are indebted for a large 

 body of knowledge in regard to the phenomena of radiation. In the 

 prosecution of his researches he had to rely upon himself. He devised 

 his own apparatus, and applied it in his own way. To produce 

 radiating surfaces, he employed metallic cubes, which to the present 

 hour are known as Leslie's cubes. The difierent faces of these cubes 

 he coated with difierent substances, and filling the cubes with boiling 



