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LECTURE XVII. 



not free access. In this way the necessary supply of carbon would be cut off from 

 the experimental plant; and without this no formation of vegetable substance 

 whatever can take place, since all organic compounds contain considerable quantities 

 of carbon. After these considerations, some readers might be in doubt as to 

 whether the great masses of carbon accumulated year by year over the whole 

 surface of the globe in the form of plant-substance are really contained in the 

 atmosphere. Nevertheless, one may be satisfied of this: the carbon dioxide in 

 the whole terrestrial atmosphere is to be reckoned, not by litres, but by thousands of 

 cubic miles; and, as has long been calculated, would provide the total vegetation 

 of the earth with carbon for many years, even if new supplies of carbon dioxide were 

 not continually being formed. It is, it is true, an optimistic fable to suppose that 

 the world is so perfectly arranged that all the carbon dioxide produced by the 

 respiration of animals and men exactly suffices to equalise the consumption of carbon 

 dioxide of the whole vegetation of the earth. Nevertheless, an enormous quantity of 

 carbon dioxide is continually being supplied to the atmosphere by this very respira- 

 tion of animals, as well as by the processes of decomposition, especially of dead 

 plants themselves, which are taking place everywhere, and by means of innumerable 

 springs charged with carbon dioxide, and by hundreds of smoking volcanoes, and in 

 our time also by hundreds and thousands of smoking chimneys, through which the 

 carbon of coal escapes as carbon dioxide, so that a lack of it need scarcely be 

 expected for thousands of years. An approximate idea of how enormously large 

 the condensation of the carbon of the atmosphere by the assimilation of plants 

 must have been hitherto, is obtained on reflecting that the deposits of coal, lignite, 

 and turf, spread over the whole earth, and the bituminous substances as great or even 

 greater in quantity which permeate mountain formations, besides asphalt, petroleum, 

 &c., are products of the decomposition of earlier vegetations, which in the course of 

 millions of years have taken from the atmosphere the carbon contained in these 

 substances and transformed it into organic substance. 



We have not yet done, however, with the consideration of our experimental 

 plant. Any one conducting such an experiment, and not prepared with a knowledge 

 of vegetable physiology, might possibly for convenience, or for some other reason, 

 place the experimental plants with their nutritive solution at the back of a chamber, 

 or in the middle of an ordinary dwelling-room or laboratory. He would then find, 

 after a few weeks, that the plants had grown it is true to some extent, and even 

 that a few fresh leaves had been formed after germination. These plants would, 

 however, look unhealthy, shrivel up, and die ; and after drying them and weighing 

 their organic substance, it would be found that the latter, according to circumstances, 

 is scarcely so large as the weight of the seeds employed, and may be considerably 

 smaller. That plants cannot flourish under such circumstances is self-evident, how- 

 ever, on learning from the researches of Ingenhouss and De Saussure, as well as 

 from our own experiments, that the decomposition of the atmospheric carbon 

 dioxide in the green parts, and the consequent production of new plant-substance, 

 only occurs when the organs containing chlorophyll meet with light of sufficient 

 intensity. By this is to be understood in general, so far as vigorous vegetation 

 is concerned, all the daylight reflected from, the sky, or, in the case of plants 

 growing entirely in the open, direct sunhght. The whole nature of the plant is 



