240 REPORT—1852. 
decomposed by the plant, it was natural enough that those who observed the 
wonderful powers of the chemical rays of the solar spectrum in reducing 
salts of silver and other substances, should refer the chemical changes taking 
place within the vegetable tissues to the same occult agency. But Dr. 
Daubeny, in an admirable investigation, published in the Philosophical 
Transactions for the year 1836, has shown by indisputable evidence that it 
is the luminous, and not the chemical or the calorific rays, which cause the 
decomposition of carbonic acid with emission of oxygen from the leaves, the 
formation of chlorophyl, the irritability of the Mimosa, the evolution of water, 
and indirectly at least the absorption of it by the roots. The colours of 
flowers are supposed by this author to depend also on the luminous rays; 
sunshine was found to act far more energetically than diffused daylight ; 
while no colouring effects were observed to be produced by any artificial 
illumination, not even by that afforded by incandescent lime. 
That portion of the inquiry requested by the British Association which 
devolved upon me, embraced a question not included in the investigations of 
the Oxford Professor, nor in those of any other experimentalist, as far as I 
am aware: I refer to the influence of various atmospheric conditions in 
conjunction with light. This circumstance necessitated the employment of 
closed vessels under whieh the plants should be grown; and glass, from its 
transparency, was not only the best but almost the sole article which could 
beemployed. Bell-shaped glasses were accordingly procured ; and they were 
made of various colours, in order that the different properties of the spectrum 
might be to a considerable extent separated. 
The blue glasses mentioned in this paper had each a capacity of about 690 
cubic inches. A smaller bell-glass of 172 inches capacity was also employed 
in an experiment not described on the present occasion. The yellow glasses 
had a capacity of 650 cubic inches; the red of 558; and those made of 
colourless glass of 740 cubic inches. The darkened glasses mentioned below 
were made by partially covering bell-jars with brown paper, thus excluding 
the light except such as passed through about one-eighth of the surface of 
the glass, and that on the side turned away from the window. Their cubic 
contents were 500 inches. Small colourless and yellow glasses were also 
procured, each having a capacity of 177 cubic inches. 
The blue glass employed is of so intense a colour, that it cuts off by far 
the greater portion of the luminous rays, but photographic paper showed that 
it admits the chemical rays freely ; it may also be considered as interfering 
much with the transmission of heat. The red glass, on the contrary, freely 
admits the calorific influence, but stops the chemical, whilst, like the blue, it 
diminishes greatly the luminous. The yellow again scarcely decreases the 
illuminating power of light, but almost destroys its chemical action. 
The place in which the experiments here described were conducted, was a 
room in a dwelling-house at Stockwell, in the neighbourhood of London. 
The glasses stood on a table close by the window, which had a S.S.E. aspect. 
No fire was ever lighted in the room, but it must have been a little warmer 
than the external atmosphere in the winter time on account of the vicinity of 
heated apartments. 
As preliminary experiments, merely the effect of these coloured glasses in 
accelerating or retarding the growth of various kinds of plants was tried. 
Hyacinths were chosen as samples of bulbous-rooted plants. They were 
all of the same description, purple in colour, as nearly as possible of the same 
size, healthy, and beginning to put forth a plumule and radicles. They were 
weighed, placed on the top of colourless glasses containing sufficient pure 
water just to touch the rootlets, and then covered with the large glass shades. 
