THE PHYSICS OP A SUNBEAM. 
439 
Chloride of silver — the salt which we have supposed was the 
chemical compound employed — is one of the most stable cha- 
racter. So great is the affinity of chlorine for silver, that it 
will combine with, and separate that metal from, almost all its 
combinations. Yet the moment it is exposed to sunshine it is 
decomposed; chlorine is liberated and metallic silver is left 
behind. Such is the remarkable phenomenon ; thus teaching’ 
us that we have to deal with agencies in the solar rays which 
are in them visible effects very dissimilar. It became neces- 
sary, when the investigations of science were directed to these 
peculiarities of solar chemistry, to distinguish by a name the 
agent which was at work. Actinism, a term signifying ray- 
power, was proposed, and it has been generally adopted to 
express the chemical principle of the sunbeam. The relations 
of light, the luminous 'power of the sun ; of heat, the calorific 
power, and of actinism, the chemical power, will now become 
the subject of consideration. 
The plate accompanying this paper shows three spectral 
images ; which, as they will serve to guide the reader in under- 
standing the somewhat involved phenomena, should be care- 
fully examined as we proceed. 
The solar spectrum, which in its chromatic relations has been 
described in a former paper, must now be considered in connec- 
tion with its illuminating power. The ray which has the highest 
illuminating power is the yellow. In either direction the 
light declines from this point. At the least refrangible end 
illumination ceases with the extreme red ray, and at the most 
refracted end there is no ordinary light beyond the violet ; or, 
speaking with philosophical exactness, beyond the lavender 
ray, which terminates the violet, fading off gradually and beau- 
tifully into darkness : there is no illuminating power. Now the 
maximum of Light and of Heat are not coincident. Sir William 
Herschel and Sir Henry Englefield long since determined that 
the maximum of Heat was to to be found in the least refrangible 
rays. Sir John Herschel has a beautiful experiment by which 
this is proved, and some new truths added to our knowledge. 
A piece of silver paper is carefully stretched upon a frame, and 
being held above a very smoky candle or lamp it is blackened 
over on one side. After having been thus prepared this screen is 
placed so that the solar spectrum can be received upon the un- 
hlackened side. When a very perfect image has been obtained by 
an accurate adjustment of the prism, and the rays are seen in all 
their beauty of colour, well defined; the paper is thoroughly satu- 
rated with ether, which should be applied with a wide flat brush. 
The tissue-paper being wetted appears perfectly black, but, in 
a very short time, the heat-rays drying up this volatile fluid, a 
curious image is produced which is a picture indeed of the dis- 
2 H 2 
