Aug. 4, 1870] 
NATURE 
271 
to produce this? We reply, the tissues of his body are 
consumed. If he continues working for a long time 
he will wear out these tissues and nature will call for food 
and rest ; for the former in order to procure the materials 
out of which new and energetic tissues may be con- 
structed ; for the latter, in order to furnish time and leisure 
for repairing the waste. Ultimately, therefore, the energy 
of the man is derived from the food which he eats, and if 
he works much, that is to say, spends a great deal of 
energy, he will require to eat more than if he hardly works 
at all. Hence it is well understood that the diet af a 
man sentenced to imprisonment with hard labour must 
be more generous than that of one who is merely im- 
prisoned, and that the allowance of food to a soldier in 
time of war must be greater than in time of peace. 
In fact, food is to the animal what fuelis to the engine, 
only an animal is amuch more economical producer of 
work than an engine. Rumford justly observed that we 
shall get more work out of a ton of hay if we give it as 
food to a horse than if we burn it as fuel in an engine. 
It is in truth the combustion of our food that furnishes 
our frames with energy, and there is no food capable of 
nourishing our bodies which, if well dried, is not also 
capable of being burned in the fire. Having thus traced 
the energy of our frames to the food which we eat, we 
next ask whence does this food derive its energy. If we 
are vegetarians we need not trouble ourselves to go 
further back, but if we have eaten animal food and have 
transferred part of the energy of an ox or of a sheep into 
our own systems, we ask whence has the ox or the sheep 
derived its energy, and answer, undoubtedly, from the 
food which it consumes, this food being a vegetable. 
Ultimately, then, we are led to look to the vegetable 
kingdom as the source of that great energy which our 
frames possess in common with those of the inferior 
animals, and we have now only to go back one more step 
and ask whence vegetables derive the energy which they 
possess. 
In answering this question, let us endeavour to ascer- 
tain what really takes place in the leaves of vegetables. 
A leaf is, in fact, a laboratory in which the active agemt 
is the sun’s rays. A certain species of the solar ray enters 
this laboratory, and immediately commences to decom- 
pose carbonic acid into its constituents oxygen and car- 
bon, allowing the oxygen to escape into the air while the 
carbon is, in some shape, worked up and assimilated. 
First of all, then, in this wondrous laboratory of Nature, 
we have a quantity of carbonic acid drawn in from the 
air: this is the raw material. Next, we have the source 
of energy, the active agent: this is light. Thirdly, we 
have the useful product : that is, the assimilated carbon. 
Fourthly, we have the product dismissed into the air, 
and that is oxygen. 
We thus perceive that the action which takes place ina 
leaf is the very reverse of that which takes place in an 
ordinary fire. In a fire, we burn carbon, and make it 
unite with oxygen in order to form carbonic acid, and 
in so doing we change the energy of position derived from 
the separation of two substances having so great an 
attraction for each other as oxygen and carbon, into the 
energy of heat. In a leaf, on the other hand, these two 
strongly attractive substances are forced asunder, the 
powerful agent which accomplishes this being the sun’s 
rays, so that it is the energy of these rays which is trans- 
formed into the potential energy or energy cf position 
represented by the chemical separation of this oxygen and 
carbon. The carbon, or rather the woody fibre into 
which the carbon enters, is thus a source of potential energy, 
and when made to combine again with oxygen, either 
by direct combustion or otherwise, it will in the 
process give out a deal of energy. When we burn wood 
in our fires we convert this energy into heat, and when we 
eat vegetables we assimilate this energy into our systems, 
where it ultimately produces both heat and work. We 
are thus enabled to trace the energy of the sun’s rays 
through every step of this most wonderful process : first 
of all building up vegetable food, in the next place feeding 
the ox or sheep, and lastly through the shape of the very 
prosaic but essential joint of beef or mutton entering into 
and sustaining these frames of ours. 
We are not, however, quite done yet with vegetable 
fibre, for that part of it which does not enter into our 
frames may, notwithstanding, serve as fuel for our 
engines, and by this means be converted into useful 
work. And has not Nature, as if anticipating the 
wants of our age, provided an almost limitless store of 
such fuel in the vast deposits of coal, by means of which 
so large a portion of the useful work of the world is done? 
In geological ages this coal was the fibre of a species of 
plant, and it has been stored up as if for the benefit of 
generations like the present. 
But there are other products of the sun’s rays besides 
food and fuel. The miller who makes use of water-power 
or of wind power to grind his corn, the navigator who 
spreads his sail to catch the breeze, are indebted to our 
luminary equally with the man who eats meat or who 
drives an engine. For it is owing to the sun’s rays that 
water is carried up into the atmosphere to be again pre- 
cipitated so as to form what is called a head of water, 
and it is also owing to the sun’s heat that winds agitate 
the air. With the trivial exception of tidal energy all the 
work done in the world is due to the sun, so that we must 
look to our luminary as the great source of all our 
energy. 
Intimately linked as we are to the sun, it is natural to 
ask the question, Will the sun last for ever, or will he also 
die out? There is no apparent reason why the sun should 
form an exception to the fate of all fires, the only difference 
being one of sizeandtime. It is larger and hotter, and will 
last longer than the lamp of an hour, but it is nevertheless 
alamp. The principle of degradation would appear to hold 
throughout, and if we regard not inere matter but useful 
energy, we are driven to contemplate the death of the 
universe. Who would live for ever even if he had the 
elixir of life? or who would purchase, if he might, the 
dreary privilege to preside at the end of all things—to be 
“twins in death” with the sun, and to fill up in his own 
experience the melancholy dream of the poet,— 
The sun's eye had a sickly glare 
The stars with age were wan, 
The skeletons of nations were 
Around that lonely man, 
Some died in war, the iron brands 
Lay rusting in their bony hands, 
In peace and famine some. 
Earth’s cities had no sound nor tread, 
And ships lay dri.ting with their dead 
‘Yo shores where all were dumb, 
B. STEWART 
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