190 LECTURES TO SCIENCE TEACHERS. 



Professor Balfour Stewart, who showed that although rock- 

 salt is a substance extremely transcalent to ordinary rays, 

 yet it is remarkable for the absorption of the rays emitted 

 by itself. A plate of warm rock-salt gives a radiation 

 which is entirely absorbed by a plate of cool rock-salt, 

 although the cool rock-salt would transmit a very large 

 percentage of any other kind of radiation. This fact may 

 be also seen in the various experiments which are more or 

 less familiar to you all. If you take a tile, or a porcelain 

 plate, on which there is a black pattern on a white ground, 

 and heat that plate to whiteness, the pattern becomes 

 reversed. The black portions of the plate which absorb 

 most light are also capable of giving out most light ; and 

 hence they appear light upon a dark ground. A still 

 simpler experiment is to take a piece of platinum foil and 

 write upon it with ink. If then you heat this platinum 

 foil to redness, the word which you have written upon it 

 in ink appears to be bright upon a dark ground ; that is 

 to say, the black ink which absorbs most of the radiation 

 is also capable of emitting most, and hence the word 

 appears brighter than the surrounding platinum. You 

 also find that the appearance behind is reversed. The 

 greater radiation from the blackened surface on the pla- 

 tinum robs the platinum of some of its heat, w and hence, 

 although the word appears to be bright in front, yet, owing 

 to the loss of heat by radiation in front, the platinum 

 appears on the other side darker where the word is written 

 than in the surrounding part. 



Here, then, we have the reciprocity of radiation and 

 absorption shown in the case of molecular vibrations ; and 

 this reciprocity is, as you are aware, the foundation of 

 spectrum analysis. For example, cool sodium vapour 

 absorbs the radiation of glowing sodium vapour. You are 

 all probably aware of the names which cluster around this 

 generalization of Kirchhoff. It has, however, always 

 seemed to me that the name of Professor Tyndall ought, 

 in justice, to be associated with the earlier workers at the 

 law which underlies the subject of spectrum analysis. At 

 the period of Kirchhoff's discovery, Professor Tyndall was 

 at work on the absorption of heat by gases and vapours. 

 The apparatus which he employed is exhibited in the Loan 

 Exhibition, and I have it here. He used as a source of 



