160 
DR. DAUBENY ON THE ACTION OF LIGHT UPON PLANTS, 
sible to command an uniform intensity of solar radiation during the whole period 
occupied by any one series of experiments, another plant of the same kind and size 
M r as placed under transparent glass ; and from the comparative amount of moisture 
emitted by it, I calculated what might be the difference in the amount of solar in- 
fluence during the period at which the experiments were carried on. 
Now although the experiments conducted on the above plan in general tended to 
show, that the extrication of moisture, cceteris paribus , was most abundant in pro- 
portion to the intensity of the light admitted, (orange glass in general causing more 
moisture to be exhaled than red or green,) yet in some instances blue and purple 
glasses, and still more remarkably, bottles filled with the cupreous solution, would 
cause a more abundant exhalation than orange or even transparent glass. Here, 
however, another principle seems to come into play, namely, the influence of heat 
radiated from the surface of the screen. This I infer, first, because the quantity 
of water exhaled under the influence of the copper solution became greatest, when the 
state of the weather was such as to elevate the temperature of the liquid considerably 
above that of the surrounding atmosphere ; and, secondly, because a bottle filled with 
water blackened with ink to such a degree, as to transmit just as much light, so far 
as could be measured by the eye, as that filled with the copper solution was found to 
do, caused an equally considerable amount of water to be evolved by the plant. 
Thus I selected two plants of the Tree Mallow ( Lavatera arborea ), which, by a 
previous experiment, had been found to exhale in the open air equal quantities of 
moisture, and placed the one under a frame, into which were inserted the bottles of 
ink and water, and the other under one with the solution of ammonio-sulphate of 
copper. Both fluids soon acquired in the sun a temperature from 110° to 120° Fahr. ; 
and at the end of two hours the sulphuric acid in connexion with each of the plants 
was successively weighed, and the increase found to be nearly uniform ; that under 
the ink and water having gained 150 grains per hour, that under the cupreous solu- 
tion 162 grains in the same time. 
Now as water with the addition of a little ink is known to absorb the rays pro- 
ceeding from all parts of the spectrum in an equal ratio, it follows, that the effect 
produced in either instance must be ascribed to the heat radiated, and not to any 
peculiar virtue of the violet extremity in stimulating the vegetative functions. 
Yet it is curious that the presence of some light seems essential to the due con- 
tinuance of this process. The same plants which had been employed in the preceding 
experiment were placed out in the sun on a bright day; one, as before, under the 
influence of the light transmitted through the cupreous solution, the other under a 
frame covered over with blue tiles, which, together with the liquid, soon became 
heated by the sun’s rays to the temperature of 110° or 120° of Fahrenheit. At the 
end of a certain time the sulphuric acid contained in the tin vessel which had 
inclosed the plant exposed to the action of the violet end of the spectrum had gained 
at the rate of 159 grains in the hour, (which is within 3 grains of the amount oh- 
