480 MR. J. B. LAWES, DR. GILBERT, AND DR. PUGH ON 
in the open air, and were protected from rain, or the too powerful rays of the sun, by a 
canvas awning which could be drawn down over them, or withdrawn, at pleasure. 
In 1858, in addition to the sets of apparatus above described, two glazed cages, such 
as were used by M. G. Vine in his experiments, and which he kindly sent over to us 
for the purpose, were employed. 
J.—The supply of Carbonic Acid to the Plants. 
Owing to the small proportion of carbonic acid in the atmosphere, and to the fact 
that a part of it would be absorbed in passing through the apparatus just described, it 
was necessary to give a supply of it to the plants artificially. It was obtained by the 
action of chlorhydric acid upon fragments of marble in the vessel T, Plate XIIL, or T’, 
fig. 1, Plate XTV. 
In regulating the supply of carbonic acid, the points to be observed were, to keep the 
proportion in the enclosed atmosphere below that in which it would prove injurious to 
the plants, and at the same time to provide a sufficient quantity for the demands of 
vegetation at the most active periods of growth. 
Bovssincautt found* that the air surrounding a plant might, consistently with 
healthy growth, contain 8 per cent. of carbonic acid. This amount, then, on the one 
hand, and the very small quantity in the atmosphere which is sufficient for natural 
vegetation (about 0-04 per cent.), on the other hand, afford us limits between which a 
wide range is allowed for variation. 
Calculation showed that a minimum quantity of 0-2 per cent. of carbonic acid in the 
air of the enclosing apparatus would supply 5 cubic inches of the gas within the shade 
at one time, corresponding to 0:0439 gramme of carbon—a quantity which, maintained 
daily throughout the sunlight, would be very much more than was required by the plants. 
It is obvious, therefore, that a variation in the amount of carbonic acid in the atmo- 
sphere of the plants between 4:0 per cent. and 0-2 per cent. would be very safely within 
the limit suggested by the experiment of BoussincauLT as the maximum, on the one 
hand, and that indicated by the above considerations as the minimum desirable in the 
experiment, on the other. 
A question arises as to the influence which the changes in the proportion of carbonic 
acid in the air, between the assumed limits, may have upon the plant. In reference to 
this point, it may be mentioned that our own experiments upon the nature of the gas 
in plants (some of the results of which will be given further on) appear to show that 
the changes in the proportion of the carbonic acid in the air of the cells and intercellular 
passages, and in that in the fluids of the stem, are much greater, and more rapid than 
those which can take place in the atmosphere of our apparatus. In addition to this, 
may be stated the fact that plants derive much of their carbonic acid from aqueous 
solution absorbed by the roots; and most probably the remainder is absorbed by the 
fluids of the plant before influencing its growth. These absorptions can take place but 
* Mémoires de Chimie Agricole et de Physiologie, 1854, p. 441. 
