SECT. XXIV. DISTRIBUTION OF CHEMICAL ENERGY. 207 



found to be unimpressable. Nevertheless, M. Niepce' de Victor 

 obtained beautiful results upon glass coated with albumen mixed 

 with sensitive substances, which suggested the medium by means 

 of which the art has been brought to its present perfection, and 

 that final step is due to Mr. Scott Archer. He coated a plate 

 of glass thinly with collodion, that is, gun-cotton dissolved in 

 ether and alcohol, which dries into a delicate transparent film 

 of extreme adhesiveness, and of such intense sensibility that the 

 action of light upon it is so instantaneous that it arrests a stormy 

 sea or a fleeting cloud before they have time to change. Now 

 landscapes in chiaroscuro are produced of great beauty, which 

 by the slower methods were mere masses of deep shade and 

 broad light. Architecture is even more perfectly obtained, 

 but it fails to give a pleasing representation of the human 

 countenance. 



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, properties most im- 

 portant 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 untrodden. 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 surface on 

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

 paper prepared with bromide of silver, the chemical spectrum, 

 as indicated merely by the length of the darkened part, includes 

 within its limits the whole luminous spectrum, extending in 

 one direction far beyond the extreme violet and lavender rays, 

 and in the other down to the extremest red : with tartrate of 

 silver the darkening occupies not only all the space under the 



