SECT. XXIV. PARATHERMIC RAYS. 217 



It appears from the experiments of MM. Becquerel and Biot, 

 that electrical disturbances produce these phosphorescent effects. 

 There is thus a mysterious connexion between the most refran- 

 gible rays and electricity which the experiments of M. E. Bec- 

 querel confirm, showing that electricity is developed during che- 

 mical action by the violet rays, that it is feebly developed by the 

 blue and indigo, but that none is excited by the less refrangible 

 part of the spectrum. 



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 refrangible 

 visible colours, and have been named by their discoverer Para- 

 thermic 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 Herschel found that in 

 experiments with a solution of gum guaiacum in soda, which 

 gives the paper a green colour, the green, yellow, orange, and 

 red rays of the spectrum invariably discharged the colour, while 

 no effect was produced by the extra-spectral rays of heat, 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 dis- 

 solved 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 

 discharged 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 un- 

 changed ; while the photographic impression within the visible 

 spectrum increased in intensity; the non-luminous thermic rays, 

 though evidently active as to heat, were yet incapable of effecting 

 that peculiar chemical change which other rays of much less 

 heating power were all the time producing. Sir John having 

 ascertained that an artificial heat from 180 to 280 of Fahrenheit 

 changed the green tint of gum guaiacum to its original yellow 

 hue when moist, but that it had no effect when dry, he there- 



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