5 H HISTORY OF SCIENCE. 



of sunshine. The whole question was reviewed by Saussure, who 

 made some additions to the facts already known. The French che- 

 mists, Dumas and Boussingault, about 1840 applied exact chemical 

 analyses to the determination of the action of vegetables with regard 

 to air, water, and manure, and to the chemical relations of animals 

 to the air and to their food. The result of these researches was 

 the confirmation of the facts that plants decompose carbonic acid 

 derived from the atmosphere, liberating the oxygen and appropria- 

 ting the carbon, while animals cause the combination of carbon and 

 oxygen. It was found that only the green parts of plants have 

 this power, and only under the influence of sunlight, and it has been 

 supposed that chlorophyl, the green colouring matter of plants, is in 

 some way specially concerned in the decomposition of the carbonic 

 acid. It was discovered by Mr. Stokes that chlorophyl exhibits re- 

 markable absorption bands in its spectrum (page 483), and that be- 

 sides this it exhibits peculiar fluorescent qualities. Dr. Draper of New 

 York placed in the several rays of a solar spectrum narrow tubes, each 

 containing a single blade of grass immersed in water charged with 

 carbonic acid. The spectrum was rendered stationary by means of a 

 heliostat, an instrument in which a mirror is so moved by clockwork 

 that the sun's rays are reflected in one fixed direction, notwithstanding 

 the change of the sun's apparent position. The quantities of oxygen 

 collected in the different rays in one of Draper's experiments were as 

 under : 



In the red 0^33 cubic centimetres. 



,, red and orange . 20 'oo 



yellow and gn 

 green and blu< 



reen 36 'oo 

 lue . o'io 



blue .... o'oo 

 violet . .o'oo 



The chemical separation of carbon and oxygen implies the expendi- 

 ture of energy, and for the source of this energy we must look to the 

 sun's rays. The energy they supply is stored up in the chemical sepa- 

 ration, and when the vegetables are consumed as food by animals, 

 these last re-combine the carbonaceous materials with atmospheric 

 oxygen, whereupon tjie stored-up energy reappears as animal heat and 

 in various physiological and mechanical forces. The animal world is 

 thus dependent for its existence on the vegetable world, and were the 

 latter removed for a single year from the face of the earth, the animal 

 life of our planet would necessarily disappear. 



The nineteenth century has witnessed not only the vast accessions 

 to our knowledge of light which have been in part indicated in a pre- 

 ceding chapter, but optical instruments have been the subject of so 

 many improvements, that microscopes and telescopes, of an excellence 

 which 100 years ago would have been considered quite unattainable, 

 are now within the reach of the most moderate means. The obliga- 

 tions of physiology and anatomy to the modern compound microscope 



