BEYOND THE RED IN THE SPECTRUM — BABCOCK 167 



were near-by but shaded from the spectrum. By readinj]j all throo 

 at rep:ular intervals of time he found that the exposed th(>rinoin<'ter 

 rose at a faster rate tiian the others. Pushing the tliermonii'tois 

 along the spectrum, he explored a considerable extent of the new 

 refrion whicli he had discovered, and ^vas able to learn something 

 of the distribution of energy in it. 



Later, by an improvement of this method of study. Sir John 

 Hersehel, son of Sir William, continued the examination of the 

 invisible spectrum. He concluded tiiat one-half of the sun's heat 

 is found in the infra-red part of the spectrum — a result surprisingly 

 close to the modern figure of 45 per cent. The important result 

 of the Herschels' work was the demonstration that visible radiation 

 is only a part of the spectrum, and that nonluminous hot bodies 

 have spectra just as those which we can see by their own light. 



Leaving the work of the Herschels, on whicli it would be easy to 

 spend all the time available here, we shall next consider the contribu- 

 tions of Professor Tyndall, the famous English physicist who lived 

 in the middle of the last century. AYith apparatus far more delicate 

 than that of his predecessors (though it would appear crude to us 

 to-day) he devoted years to a thorough study of the phenomena of 

 heat, covering both the visible and the infra-red parts of the 

 spectrum. To give an adequate idea of the scope and the importance 

 of Tyndall's brilliant experimental work would take far too long. 

 It furnished a sure foundation of established facts on which later 

 the great theoretical treatment of radiation was to be laid, and it 

 still constitutes a rich fund of information. _ Tyndall studied, among 

 many other things, the absorbing power of gases for radiation. I 

 think he was the first to recognize the remarkable difference in the 

 absorbing power exhibited by a mixture of certain molecules and by a 

 chemical combination of the same molecules. 



For example, if a certain number of molecules of oxygen are 

 mixed with twice as many of hydrogen, the power of tlic mixture to 

 absorb infra-red radiation is very small. But if these same molecules 

 are combined chemically to form water vapor, the absorptive power 

 is much greater. This fact finds a unique explanation in the modern 

 theory of molecular structure. 



Another problem which Tyndall undertook to investigate was to 

 find the proportion in which the total energy radiated by a hot body 

 is divided between visible and obscure portions of its spectrum. 

 This proportion is known as the luminous efficiency. For example, 

 we have already seen that in the case of the sun the luminous effi- 

 ciency is roughly 50 per cent. Tyndall thus greatly extended the 

 study of the distribution of energy in the spectrum of a hot body, 

 which had been begun by Sir William Herschel. One of Tyndall's 



