LIGHT. 103 



from the first plate, and secondly when it has been turned 

 round through an arc of 90, and the polarised beam is 

 absorbed. I expected that, if the experiment were carefully 

 performed, the temperature of the second plate would be more 

 raised in the second case than in the first, and that it might 

 afford interesting results when tried with light of different 

 colours. I met with difficulties in procuring a suitable ap- 

 paratus, and was endeavouring to overcome them, when I 

 found that Knoblauch had, to some extent, realised this result. 

 He finds that, when a solar beam, polarised in a certain 

 plane, is transmitted perpendicularly to the axis of a crystal 

 of brown quartz or tourmaline, the heat is transmitted in a 

 smaller proportion than when the beam passes along the 

 direction of the axis of the crystal. 



It is generally as far as I am aware, universally true 

 that, while light continues as light, even though reflected or 

 transmitted by different media, little or no heat is developed; 

 and, as far as we can judge, it would appear that, if a medium 

 were perfectly transparent, or if a surface perfectly reflected 

 light, not the slightest heating effect would take place ; but, 

 wherever light is absorbed, then heat takes its place, affording 

 us, apparently, an instance of the conversion of light into 

 heat, and of the fact that the force of light is not, in fact, 

 annihilated, but merely changed in character, becoming, in 

 this instance, converted into heat by impinging on solid 

 matter, as in the instance mentioned in treating of heat, this 

 force was shown to be converted into light by impinging on 

 solid matter. But the different effects of different substances 

 in transmitting and reflecting heat and light make this diffi- 

 cult of proof. One experiment, indeed, of Melloni, already 

 mentioned, would seem to show that light may exist in a 

 condition in which it does not produce heat, which our 

 instruments are able to detect ; but some doubt has recently 

 been thrown on the accuracy of this experiment ; probably 

 the substances themselves through which the light is trans- 

 mitted would be found to have been heated. 



The recipient body, or that upon which light impinges, 

 seems to exercise nearly as important an influence on our 

 perceptions of light as the emittent body, or that from which 



