392 Prof. Draper on the Chemical Action of Light. 



These considerations enable us to explain what takes place in 

 the oeconomy of plants. The water of the soil is always charged 

 with carbonic acid, which communicates to it the quality of dis- 

 solving bone-earth ; the solution passing through the spongioles 

 goes to the leaves as ascending sap. Here it is exposed to the light, 

 the effect of which is, aided by the cell-growth there taking place, 

 to set the ])hosphoric acid free, and turn its phosphorus into the 

 passive state. Its continued u.nion with oxygen as an acid com- 

 pound thus becomes impossible, and it is now associated with the 

 proteine and oily bodies forming in the plant. Nor does it again 

 unite with oxygen until it has passed into the systems of animals 

 as a constituent of their nervous and muscidar tissues. At the 

 moment of activity of these, and especially of the former, it is 

 oxidized ; the change being apparently an immediate consequence 

 of that activity, and reverting to the acid state, it is finally dis- 

 missed from the oeconomy under the form of phosphate of soda 

 and ammonia. 



In the same manner may be exj^lained the decomposition of 

 carbonic acid by plants in the sunshine ; for carbon, like phos- 

 phorus, and indeed like all other elementary bodies, has its active 

 and passive states, as is exemplified in the contrast between dia- 

 mond and lamp-black. Tlie sunlight enables the leaves of plants 

 to bring the carbon into the inactive state, and decomposition 

 ensues as a secondaiy result. The resulting carbon compounds 

 form the food of various animals ; nor does this element recover 

 its active state until it has given rise to the processes of life, 

 when it suddenly unites with oxygen brought by the arterial 

 blood, and the carbonic acid and cyan ate of ammonia are dij§-" 

 missed from the system by the lungs and kidneys conjointly. ' 



At first sight it might appear that the mechanism of decofn- 

 position by simple vibratory movement is essentially different 

 from that by these allotropic changes. But this is not neces- 

 sai-ily so. Thus the passage of iron from the active to the pas- 

 sive state is probably connected with the equivalency of two 

 atoms of it to one of the chlorine group, so that a pair of atoms • 

 may replace a solitary one. The association of similar atoms in 

 pairs is a natural consequence of the establishment of vibratory ' 

 motions in a given group. ';;([ ^''>f<^ ^' '*'''■' '-'* '^ ^^ 



,11 lu^noiioai :■■ <, . ■ 'rui'Iiib uH 



jBiit lis; 81 , JJiuii I ' lo 'idt "H. 



In conclusion it may be observed, that I have cndeavoiii'6'd' t^^ 

 investigate in this memoir how far the decomposing action of a 

 ray is dependent on the amplitude, or frequency, or direction of 

 its vibrations. The result aiTived at is, that decompositions are 

 not determined by amplitude ; since a faint light continued long 

 enough can produce precisely the same effect as the more con- 



