SECT, xxv.] CONCLUSIONS TO BE DEAWN. 267 



certain temperature must be attained, and that suffices in total 

 darkness : yet, on exposing to a very concentrated spectrum a 

 slip of the paper usedinthe last experiment, after the uniform 

 blue colour has been discharged and a white ground left, this 

 whiteness is changed to brown over the whole region of the 

 red and orange rays, but not beyond the luminous spectrum. 



Sir John thence concludes 1st. That it is the heat of 

 these rays, not their light, which operates the change ; zndiy. 

 That this heat possesses a peculiar chemical quality which 

 is not possessed by the purely calorific rays outside of the 

 visible spectrum, though far more intense ; and, Srdly. That 

 the heat radiated from obscurely hot iron abounds especially 

 in rays analogous to those of the region of the spectrum above 

 indicated. 



Another instance of these singular transformations may be 

 noticed. The pictures formed on cyanotype paper rendered 

 more sensitive by the addition of corrosive sublimate are blue 

 on a white ground and positive, that is, the lights and shadows 

 are the same as in nature, but, by the application of heat, the 

 colour is changed from blue to brown, from positive to nega- 

 tive ; even by keeping in darkness the blue colour is restored, 

 as well as the positive character. Sir John attributes this, as 

 in the former instance, to certain rays, which regarded as rays 

 of heat or light, or of some influence mi generis accompanying 

 the red and orange rays of the spectrum, are also copiously 

 emitted by bodies heated short of redness. He thinks it 

 probable that these invisible paratherinic rays are the rays 

 which radiate from molecule to molecule in the interior of 

 bodies, that they determine the discharge of vegetable colours 

 at the boiling temperature, and also the innumerable atomic 

 transformations of organic bodies which take place at the 

 temperature below redness, that they are distinct from those 

 of pure heat, and that they are sufficiently identified by these 

 characters to become legitimate objects of scientific discussion. 



The calorific and parathermic rays appear to be so inti- 

 mately connected with the discoveries of Messrs. Draper and 



