232 EXPERIMENTS ON LIGHT AND HEAT. SECT. XXV. 



gum guaiacum to its original yellow hue when moist, 

 but that it had no such effect when dry, he therefore 

 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 accelerated 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 ther- 

 mic, 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 circumstances 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 solu- 

 tions of ammonia-citrate of iron and ferro-sesquicyanite 

 of potash in equal parts, then thrown into water and 

 afterward 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 discharged : but if it be well washed and dried 

 and a hot smoothing iron passed over it, the picture in- 

 stantly 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 origi- 

 nal 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 concentrated spectrum a slip of the 

 paper used in the last experiment, after the uniform 

 blue color has been discharged and a white ground left, 

 this whiteness is changed to brown over the whole re- 

 gion of the red and orange rays, but not beyond the 

 luminous spectrum. 



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



