148 
POPULAR SCIENCE REVIEW. 
duced (the rate of motion attained) determines the physical 
effect, and by employing the heat thus developed to do work, 
we obtain from it the same equivalent of motion, less the loss 
which arises from the imperfection of our means for employing 
the ethereal spirit. Heat, to work like a slave in chains. 
In some previous articles * the physical conditions of a 
sunbeam have been examined, and the beautiful results de- 
pendent upon the cautious observation of the solar spectrum 
(spectrum analysis) described. Not only this, but the relations 
have been shown between the spectra obtained from artificial 
flames, the composition of their burning matter being known, 
and the solar spectrum, the composition of the Sun^s burning 
matter being previously unknown. The result being that it has 
been determined, with quite as much certainty as any other 
discovery depending upon human sense, that the conditions 
under which we can produce certain phenomena in the spectra 
of artificial flames, are actually the conditions under which we 
obtain the solar spectrum with its remarkable black lines. 
This, therefore, leads us to a knowledge of much that is going 
forward in the Sun ; and it enables us to assert, with good pre- 
sumptive evidence in favour of its correctness, that heat 
originates in that mighty orb under like conditions to those 
with which we are familiar ; that the solar radiations themselves 
afford us the evidence that they are the result of chemical or 
physical changes, like those which produce heat and light 
upon this earth. 
It becomes necessary now that the present state of our 
knowledge of the physical constitution of the sun should be 
examined. It is regretted that it is not possible to give the 
steps by which discoveries have been made, or to discuss 
the claims of all the numerous laborious observers to whom 
we are indebted for them. A general review of the subject is 
all that our space allows, and the high claims of those 
astronomers whose names appear not in this article are by 
no means disallowed. When the Sun is viewed through 
powerful telescopes, its surface — that is, the luminous envelope 
of the mass — is seen to have a peculiar mottled or curdy 
appearance. Arago proposed that this envelope should be 
called the Photosphere, a name now generally adopted. By 
the elder Herschel, the surface of this photosphere was com- 
pared to mother-of-pearl. Other astronomers have said that 
it resembles the ocean on a tranquil summer day, when its 
surface is slightly crisped by a gentle breeze, and an un- 
dulating play of light is reflected from these little billows. 
* Popular Science Eevtew, Vol. I. p. 205 and p. 438. See also “The 
Sun and Solar Phenomena/’ p. 299. 
