228 DISTRIBUTION OF CHEMICAL ENERGY. [SECT. xxrv. 



white and purple photograph results. It is vain to attempt 

 to describe 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 

 blueish purple image, that of the ten-week stock a fine rose 

 colour on a pale straw-coloured 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 preparation. Water is fre- 

 quently sufficient ; indeed Sir John Herschel brought out 

 dormant photographs by breathing on them, and some sub- 

 stances are insensible to the action of light till moistened, 

 as for example, gum guaiacum. 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 disclosed the 

 wonderful nature of the emanation. In the research, proper- 

 ties most important and unexpected have been discovered by 

 Sir John Herschel, who imprints the stamp of genius on all he 

 touches his eloquent papers can alone convey an adequate 

 idea of their value in opening a field of inquiry vast and un- 

 trodden. The following brief and imperfect account of his 

 experiments 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 extremity. This dis- 

 tribution does not depend on the refrangibility of the rays 

 alone, but also on the nature of the rays themselves, and on 

 the physical properties of the analyzing medium on which the 

 rays are received, whose changes indicate and measure their 

 action. The length of the photographic image of the same 

 solar spectrum varies with the physical qualities of the sur- 

 face on which it is impressed. When the solar spectrum is 



