MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 459 



place in the economy of plants. The water of the soil is 

 always charged with carbonic acid, which communicates 

 to it the quality of dissolving bone-earth. The solution 

 passing through the spongioles goes to the leaves as as- 

 cending sap. Here it is exposed to light, the effect of 

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

 set the phosphoric acid free, and turn its phosphorus into 

 the passive state. Its continued union with oxygen as 

 an acid compound thus becomes impossible, and it is 

 now associated with the proteine and oily bodies form- 

 ing in the plant. Nor does it again unite with oxygen 

 until it has passed into the systems of animals as a con- 

 stituent of their nervous and muscular 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 dismissed from the system under the 

 form of phosphate of soda and ammonia. 



In the same manner might be explained the decompo- 

 sition of carbonic acid by plants in the sunshine ; for car- 

 bon, like phosphorus, and, indeed, like all other elemen- 

 tary bodies, has its active and passive states, as is exem- 

 plified in the contrast between diamond and lamp-black. 

 The sunlight enables the leaves of plants to bring the 

 carbon into the inactive state, and decomposition ensues 

 as a secondary result. The carbon compounds arising 

 form the food of various animals; nor does this element 

 recover its active state until it has given rise to the proc- 

 esses of life, when it suddenly unites with oxygen brought 

 by the arterial blood, and the compounds it then forms 

 are dismissed from the system by the lun^s and kidneys 



/ / V 



conjointly. It might seem that the mechanism of decom- 

 position by vibratory movement is essentially different 

 from that by these allotropic changes, but a more detail- 

 ed examination will show that this is not necessarily so. 



