SCT. XXIV. DISTRIBUTION OP CHEMICAL ENERGY. ] 97 



of light is checked by washing the picture in pure 

 water. 



In cyanotypes, a class of photographs discovered by 

 Sir John Herschel, in which cyanogen in its combina- 

 tions with iron forms the ground, the pictures are 

 Prussian blue and white. In the chrysotype of the 

 same eminent philosopher, the image is first received 

 on paper prepared with the ammonia-citrate of iron, 

 and afterward washed with a neutral solution of gold. 

 It is fixed by water acidulated with sulphuric acid, and 

 lastly by hydriodate of potash, from which a white and 

 purple photograph results. It is vain to attempt to de- 

 scribe the various beautiful effects which Sir John 

 Herschel obtained from chemical compounds, and from 

 the juices of plants : the juice of the red poppy gives a 

 positive bluish purple image, that of the ten-week stock 

 a fine rose color on a pale straw-colored ground. 



Pictures may be made by exposure to sunshine, on 

 all compound substances having a weak chemical affinity, 

 but the image is often invisible, as in the Daguerreotype, 

 till brought out by washing in some chemical prepara- 

 tion. Water is frequently sufficient ; indeed Sir John 

 Herschel brought out dormant photographs by breathing 

 on them, and some substances are insensible to the ac- 

 tion of light till moistened, as for example gum guaia- 

 cum. Argentine papers, however, are little subject to 

 the influence of moisture. The power of the solar rays 

 is augmented in certain cases by placing a plate of glass 

 in close contact over the sensitive surface. 



Chemical action always accompanies the sun's light, 

 but the analysis of the solar spectrum has partly dis- 

 closed the wonderful nature of the emanation. In the 

 research, properties most important and unexpected 

 have been discovered by Sir John Herschel, who im- 

 prints the stamp of genius on all he touches his elo- 

 quent papers can alone convey an adequate idea of then? 

 value in opening a field of inquiry vast and untrodden. 

 The following brief and imperfect account of his exper- 

 iments is all that can be attempted here : 



A certain degree of chemical energy is distributed 

 through every part of the solar spectrum, and also to a 

 considerable extent through the dark spaces at each ex- 



