266 EXPERIMENTS ON LIGHT AND HEAT. [SECT. xxv. 



the visible spectrum increased in intensity, the non-luminous 

 thermic rays, though evidently active as to heat, were yet in- 

 capable of effecting that peculiar chemical change which 

 other rays of much less heating power were all the time pro- 

 ducing. 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 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 dis- 

 coloration over the spaces occupied by the less refrangible " 

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

 mum 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 cir- 

 cumstances and in an equal state of condensation. 



The conclusions drawn from these experiments are con- 

 firmed 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 equalparts, 

 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 discharged : 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 



