334 The Modern Aspects of Physical Science. [July, 
We have spoken of the energy, or power of doing work, as 
developed by or from a force dormant in the gunpowder. Let us 
examine this a little further. This combustible substance is a com- 
pound of carbon, sulphur, and nitre; and it owes its power ( force) 
to the collision of the molecules of the carbon with those of the 
oxygen concentrated in the nitre. Without involving ourselves in 
the mysteries of combustion—for we must call them so—we admit 
that the molecules of carbon and those of oxygen being forced into 
union, Heat and Light are the result. Did these physical forces 
exist as positive entities in the carbon or in the nitre, or were they 
created from the molecules of matter by the act of union? The 
answer is before us:—‘ In long bygone ages the energies of solar 
light and heat were occupied in the development of woody tissue ;” 
and every molecule of wood is such by virtue of the energy which 
has been expended in producing it. A cube of wood represents an 
equivalent of solar force, which, as the energies in sunshine, did the 
work of producing it (7.¢. of establishing growth), and they were 
“used up” in doing the work. The solar energies were not de- 
stroyed, but from an active they changed into a passive state; and 
the wood is a reservoir of sleeping—dormant power. Thus pro- 
bably it is with the oxygen of the nitre; but we are not yet in a 
position to trace out satisfactorily the process of the absorption of 
solar energy by any inorganic mass. If the magic and the beauty 
of photographic phenomena had not led men away from the study 
of the science of the chemical energy of the sun (Actinism) to the 
production of heliographic pictures, we might possibly have advanced 
our knowledge in this direction. 
In the process of converting the wood into charcoal, there does 
not appear to be any appreciable loss of its solar energy ; therefore 
the grains of charcoal in the gunpowder are cells of sunshine, and 
by combustion this is suddenly developed. There is, of course, some 
solar energy—Heat and Light—lost in the flash of light seen at the 
mouth of the cannon, but much of it has given motion—energy— 
to the shot, and sent it forth to do its work of destruction, in which, 
by the sudden suspension of its motion, Heat and Light are again 
seen ready to do some work somewhere: but we can trace force and 
energy no further. 
To give one other example in illustration of the great prevailing 
doctrine of our modern philosophers—coal is but changed vegetable 
matter which grew under the influences of sunshine, myriads of ages 
ere yet man had being. This substance holds its equivalent of solar 
energy. It is placed under the boiler of a steam-engine, and com- 
bustion is established. The heat passes into the water and it 
becomes steam. This water-vapour, full of solar energy, produces, 
by ingenious mechanical contrivances, motion, which is applied in 
doing work of various kinds. If, however, it is employed in driving 
