SECT, xxv.] HEESCHEL'S EXPERIMENTS. 265 



solely the effects of the action of matter upon light ; and that 

 light itself, as well as heat and sound, are not real beings, 

 but mere modes of action communicated to our perceptions 

 by the nerves. The human frame may therefore be regarded 

 as an elastic system, the different parts of which are capable 

 of receiving the tremors of elastic media, and of vibrating in 

 unison with any number of superposed undulations, all of 

 which have their perfect and independent effect. Here our 

 knowledge ends; the mysterious influence of matter on 

 mind will in all probability be for ever hid from man. 



A series of experiments by Sir John Herschel have disclosed 

 a new set of obscure rays in the solar spectrum, which seem 

 to bear the same relation to those of heat that the photographic 

 or chemical rays bear to the luminous. They are situate in 

 that part of the spectrum which is occupied by the less re- 

 frangible visible colours, and have been named by their dis- 

 coverer Parathermic rays. It must be held in remembrance 

 that the region of greatest heat in the solar spectrum lies in 

 the dark space beyond the visible red. Now Sir John Her- 

 schel found that in experiments with a solution of gum guaia- 

 cum in soda, which gives the paper a green colour, the green, 

 yellow, orange, and red rays of the spectrum invariably dis- 

 charged the colour, while no effect was produced by the 

 extra-spectral rays of caloric, which ought to have had the 

 greatest effect, had heat been the cause of the phenomenon. 

 When an aqueous solution of chlorine was poured over a slip 

 of paper prepared with gum guaiacum dissolved in soda, a 

 colour varying from a deep somewhat greenish hue to a fine 

 celestial blue was given to it ; and, when the solar spectrum 

 was thrown on the paper while moist, the colour was dis- 

 charged from all the space under the less refrangible luminous 

 rays, at the same time that the more distant thermic rays 

 beyond the spectrum evaporated the moisture from the space 

 on which they fell : so that the heat spots became apparent. 

 But the spots disappeared as the paper dried, leaving the 

 surface unchanged ; while the photographic impression within 



