154 
POPULAR SCIENCE REVIEW. 
darhiess of a solar spot ought not to exists but that we should 
look in upon a sphere intensely incandescent. In answer to 
this, it is replied : If the brilliant oxy-calcium light (the lime 
or Drummond light) be held between the eye and the sun, it 
appears as a black spot ; consequently that the blackness of a 
spot affords no evidence that intense combustion may not be 
going forward. However this may be, it must be admitted 
that the weight of the results of observation and of experiment 
goes in favour of the hypothesis that the Sun is a himiing mass 
surrounded hij vaporiforin media, circulating in all probability 
in well-defined zones, and exhibiting within themselves dis- 
similar evidences of vast energies. The inference is, that we 
derive light, heat, chemical power {actinism), and all the forms 
of electrical force from these zones of vapour. How we know 
that physical energies can only be developed from matter by 
its undergoing’ a change of form. Man can only speak of the 
conditions with which he is acquainted. To produce heat or 
light from any body is to burn that body — that is, its form is 
changed. Ordinarily, oxygen is taken from the air, and the 
body undergoing combustion becomes an oxide. We can only 
understand the combustion of iron — leading to the production 
of iron vapour — in the sun, by supposing some such change as 
this. No matter is destroyed, either on this Earth or in the 
Sun, by the processes of combustion. The oxidized or changed 
particles return back again into the orb from which they 
originated. The solid matter of the Sun cannot pass beyond 
the influence of its own gravitating force ; therefore there 
cannot be any waste of it. But in the process which develops 
those vast energies — the physical forces — they are radiated off 
into space, and a constant waste of them must be going on. Unless 
there were some provisions for restoring to the Sun its energy, 
it would in time become a dead globe of metallic ashes, and 
every planet would pass into the same state. How then are 
the physical forces restored to the central orb from which we 
derive them ? What provision is there for the permanent 
maintenance of those energies upon which heat and conse- 
quently life and light depend ? The answers to these questions 
are amongst the finest speculations of our modern philosophy. 
For the answers which have been given we are chiefly indebted 
to the inquiries and speculations of Herschel, Joule, Waterston, 
Thomson, and Mayer. It is difficult to give a brief explana- 
tion of so large a subject, but since the hypothesis which is 
comprehended within what is termed the conservation of 
energy has its foundation in the well-known law of gravita- 
tion, it may not be necessary to enter into a tedious discussion 
to render it clear to the general reader. It has been already 
said that a mass of matter falling to the Barth acquires a certain 
