28^- Mr. Hunt on Actino-Chemisiry/ 



decided colours are produced on the pure chloride of silver, — 

 a blue or fine rich lead colour, by the space between the most 

 refrangible green, and the extreme edge of the violet rays, bor, 

 yond which for some distance the same colour is produced,; 

 but gradually lowered, until it becomes a mere tint of a lilac 

 hue, at a distance beyond the visible spectrum, equal to about 

 one-third its length. Now, supposing we divide the visible 

 group of rays into 100 parts, this tinting goes on over a space 

 equal in extent to thirty-four of such parts. Over the space 

 covered by the ordinary and the extreme red rays, a very de- 

 cided red impression is slowly made. By combining the in- 

 fluence of the red and the blue rays, the chloride with tole- 

 rable quickness becomes of a fine brown colour. I find it 

 would be quite practicable to arrange a scale of colours with 

 which the actinized chloride of silver might be compared, and 

 thus enable us to ascertain with tolerable correctness the re- 

 lative quantities of heat and chemical power existing in the 

 solar rays at any time. By means of an arrangement similar 

 to that used by Mr. Shaw and Dr. Percy, by which the chlo- 

 ride is restored, after darkening, to its original condition by 

 the action of chlorine, a very accurate and interesting series 

 of observations might be made*. ,/ 



23. As it appeared probable that artificial heat would prop^ 

 duce similar effects to those described as the result of expo- 

 sing chloride of silver to the thermic rays, many experiments 

 were made with this view. In no instance however would ar- 

 tificial heat directly applied or reflected from a metallic sur- 

 face produce any change in the colour of the chloride of silver, 

 short of the point at which it is converted into horn silver, 

 and then the colour is but very slightly altered ; and even if 

 the heat is incautiously allowed to become too great, it only 

 passes into a lead colour, this change arising from the reduc- 

 tion of some portions of the chloride to the metallic state. 

 From this we may infer that the solar and terrestrial heat have 

 properties distinct from each other, or perhaps that solar heat 

 has, of itself, in its ordinary state, no particular pov/er in pro- 

 ducing a change in the chemical condition of bodies, but that 

 it modifies the action of actinic force, in a similar manner to 

 which light has been shown to do so (see the published experi- 

 ments of Herschelf, Becquerel J, Draper and the author §). 



24. Mr. Fox Talbot, and, following him, most other inqui- 

 rers, have stated that in the preparation of the ordinary chlo- 

 ridated photographic papers, it was necessary, to ensure sensi- 

 bility to solar action, that the chloride of silver should have 



7 j* Philosophical Magazine, vol. xxv. p. 450. 



-f* Philosophical Transactions, 1841. J Annalcs de Chimie, 1843. 



§ Philosophical Magazine for 1840, 1842, 1843, 1844 and 1845. 



