Another important, and even more essential fact, in connection with life, is the existence in the atmosphere of a small but nearly constant proportion of carbonic acid gas, this being the source from which the whole of the carbon in the vegetable and animal kingdoms is primarily derived. The leaves of plants absorb carbonic acid gas from the atmosphere, and the peculiar substance, chlorophyll, from which they derive their green colour, has the power, under the influence of sunlight, to decompose it, using the carbon to build up its own structure and giving out the oxygen. In the laboratory the carbon can only be separated from the oxygen by the application of heat, under which certain metals burn by combining with the oxygen, thus setting free the carbon. Chlorophyll has a highly complex chemical structure very imperfectly known, but it is said to be only produced when there is iron in the soil. The leaves of plants, so often looked upon as mere ornamental appendages, are among the most marvellous structures in living organisms, since in decomposing carbonic acid at ordinary temperatures they do what no other agency in nature can perform. In doing this they utilise a special group of ether-waves which alone appear to have this power. The complexity of the processes going on in leaves is well indicated in the following quotation:-- 'We have seen how green leaves are supplied with gases, water, and dissolved salts, and how they can trap special ether-waves. The active energy of these waves is used to transmute the simple inorganic compounds into complex organic ones, which in the process of respiration are reduced to simpler substances again, and the potential energy transformed into kinetic. These metabolic changes take place in living cells full of intense activities. Currents course through the protoplasm and cell-sap in every direction, and between the cells which are also united by strands of protoplasm. The gases used and given off in respiration and assimilation are floated in and out, and each protoplasm particle burned or unburned is the centre of an area of disturbance. Pure protoplasm is influenced equally by all rays: that associated with chlorophyll is affected by certain red and violet rays in particular. These, especially the red ones, bring about the dissociation of the elements of the carbonic acid, the assimilation of the carbon, and the excretion of the oxygen.'[12]