218 CONCLUSIONS TO BE DRAWN. SECT. XXIV. 



fore tried whether heat from a hot iron applied to the back of 

 the paper used in the last-mentioned experiment while under 

 the influence of the solar spectrum might not assist the action 

 of the calorific rays ; but, instead of doing so, it greatly acce- 

 lerated the discoloration over the spaces occupied by the less 

 refrangible rays, but had no effect on the extra-spectral region 

 of maximum heat. Obscure terrestrial heat, therefore, is capable 

 of assisting and being assisted in effecting this peculiar change 

 by those rays of the spectrum, whether luminous or thermic, 

 which occupy its red, yellow, and green regions ; while, on the 

 other hand, it receives no such assistance from the purely 

 thermic rays beyond the spectrum acting under similar circum- 

 stances and in an equal state of condensation. 



The conclusions drawn from these experiments are confirmed 

 by that which follows : a photographic picture formed on paper 

 prepared with a mixture of the solutions of ammonia-citrate of 

 iron and ferro-sesquicyanite of potash in equal parts, then thrown 

 into water and afterwards dried, will be blue and negative, that 

 is to say, the lights and shadows will be the reverse of what 

 they are in nature. If in this state the paper be washed with a 

 solution of proto-nitrate of mercury, the picture will be dis- 

 charged ; but if it be well washed and dried, and a hot smoothing- 

 iron passed over it, the picture instantly reappears, not blue, but 

 brown; if kept some weeks in this state in perfect darkness 

 between the leaves of a portfolio, it fades, and almost entirely 

 vanishes, but a fresh application of heat restores it to its full 

 original intensity. This curious change is not the effect of -light, at 

 least not of light alone. A certain temperature must be attained, 

 and that suffices in total darkness ; yet, on exposing to a very con- 

 centrated spectrum a slip of the paper used in the 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 ; 2ndly. That 

 this heat possesses a peculiar chemical quality which is not pos- 

 sessed by the purely calorific rays outside of the visible spectrum, 

 though far more intense; and, 3rdly, That the heat radiated 

 from obscurely hot iron abounds especially in rays analogous to 

 those of the region of the spectrum above indicated. 



