58 
SOLAR LIGHT AND ITS AGENCY. 
into the phenomena of actinic force." Three diagrams were then produced ; the 
first exhibited the passing of a pencil of solar light through a prism, by which 
the three primitive colours and their blendings were displayed ; the second showed 
the effect produced by the same spectrum upon blackened paper marked with ether, 
by which means it was ascertained where the heating power of the sunbeam resided ; 
a third represented the chemical change produced by the same prismatic image when 
thrown upon nitrate of silver or any of those sensitive photographic preparations which 
were susceptible of chemical change under solar influence. 
The phenomena are described " as arising from the different degrees of refrangi- 
bility in these subtile principles." These principles, it is true, are coincident with 
varying refraction ; but judging from effects, we may be pretty certain that they 
are in their nature essentially independent. As to actinism, it appears to combine 
the chemical powers of electricity and magnetism, as they exist in the solar beams. 
" Further experiments," says Mr. Hunt, " have proved that although light was 
injurious to the germinating seed, and actinism a most powerful accelerating agent 
in that process when the first leaves were developed, the actinic principle, as separated 
from light, became too stimulating, and that the luminating principle was then 
demanded to effect in the plant the decomposition of the carbonic acid inhaled by the 
leaves and absorbed by the bark, (liber), and the secretion of carbon (as the 
principle lignin), to form woody structure." The influence of heat, and its necessity 
to vegetable life, were well known ; " but it had also been proved that as the calorific 
rays increased towards autumn, the luminous and actinic relatively diminished ; the 
scorching effects to be prevented were the result of some of the heat rays, and he 
had discovered to which class they belong, by spreading the expressed juice of Palm 
leaves over paper, and exposing it to the action of the spectrum. This class of ray 
had the power of acting partly by the force of heat, and partly by chemical agency." 
It appeared to Mr. Hunt, that in adopting any tinted glass, care should be 
taken that neither light nor actinism be intercepted. The desideratum required 
was a peculiar kind of green glass which should intercept only the scorching rays, 
without obstructing those of ordinary heat-light and actinic power. Mr. Hunt, 
therefore, aided by the contractor for the Conservatory, procured green glasses of 
every variety of tint, but they were all found objectionable. He then experimented 
with fluids, diluting the colours to any degree, and examining the absorbing power 
of a great variety of chemical bodies. By this means he discovered a colour 
produced by oxide of copper, in a very diluted state, which would effectually obstruct 
the scorching rays; and Messrs. Chance and Co., (of Birmingham) after many 
trials, succeeded in producing a glass which neither obstructed light, nor interfered 
with the colour of the most delicate white flowers, nor excluded the passage of the 
chemical rays, while it would completely prevent the permeation of those heat rays, 
which were found to have so remarkable a scorching quality. In the manufacture 
of this glass, the entire absence of manganese, though used in the ordinary con- 
struction of glass, had been insisted on, because that mineral when used in the 
