442 
rOPULAli SCIENCE REVIEW. 
through it ; but it will not admit of the permeation of an Actinic 
ray. 
If; on the contrary, we take a glass coloured to a dee}) inten- 
sity of blue with oxide of cobalt, we shall find this is almost 
dark, — but little Light can pass through it. Experiment 'will, 
however, prove that it offers no obstruction to the chemical 
rays. 
How, if we place the most sensitive of photographic prepara- 
tions in a room, the Light to which is admitted through windows 
glazed with yellow glass, howsoever brilliantly the sun may shine, 
though the apartment be flooded with light, chloride, iodide, 
or bromide of silver will remain for any length of time perfect!}' 
unchanged. 
Glaze the room with cobalt-blue glass, which is so dark as to 
obscure the Lig’ht, these salts will blacken with great rapidity ; 
proving that, although the light has been obstructed, Actinism 
has suffered no interruption. 
The strength of evidence appears to be in favour of consider- 
ing Light, Heat, and Actinism as three distinct principles or 
powers, active in regulating the great phenomena of Nature. 
In the sunbeam these powers are balanced against each other, 
and thus are determined those differences of climate which are 
not influenced by the physical conformation of the earth’s 
surface. 
It has been proved by well-conducted observations, that with 
variations of latitude there are variations in the relations of these 
three principles. 
In the temperate regions of the earth the Actinic power is 
active; as Ave advance to the Tropics, where Heat increases, 
and, 
“ The sun shines for ever unchangeably bright ; ” 
the chemical power is weak. The photographic picture Avkicli 
could be taken in London in a second or tAvo, could not be 
obtained within the Tropics in less than a quarter of an hour. 
It often happens, indeed, that prolonged exposure under a 
blazing sun is insufficient to produce any chemical change. 
Everything appears to favour the view that the distribution of 
plants and animals on the surface of the earth is regulated by 
the balance of physical forces in the sunbeam. 
In the Seasons Ave detect the same influences at Avork. Ac- 
tinism or chemical poAver is greatest in the Spring; as the 
bright Light of Summer advances, the power of the Solar rays 
to produce any chemical change is diminished; and as we 
advance to Autumn, the peculiar Heat-rays come more evidently 
into action. 
The ethereal agencies at Avork in the sunbeam play a most 
