directly converted into heat, which is manifested by ignition of 
_ the wire, whereas the thermopile gives an illustration of the con- 
_ yersion of heat into electricity. ‘Che heat of combustion is the 
' reselt of the chemical combination of two substances ; but does 
it not follow from this that oxygen is a combustible as wel! as 
the carbonaceous substance which goes by the name of fuel? 
_ This is, unquestionably, the case, and if our atmosphere was 
composed of a carbonaceous gas we should haye to conduct our 
__ oxygen through tubes and send it out through burners to supply 
‘us with light and heat, as will be seen by the experiment in 
' which I burn a jet of atmospheric air in a transparent globe 
- filled with common lighting gas ; but we could not exist under 
such inverted conditions, and may safely strike out oxygen and 
~ analogous substances such as chlorine from the list of fuels. 
We now approach the second part of our inquiry—Whence is 
fuel derived ? 
The rays of the sun represent energy in the form of heat and 
light, which is communicated to our earth through the trans- 
" parent medium which must necessarily fill the space between us 
and our great Juminary. If these rays fall upon the growing 
plant, their effect disappears from direct recognition by our 
senses, inasmuch as the leaf does not become heated as it would 
if it was made of iron or dead wood, but we find a chemical 
~ result accomplished, viz., carbonic acid gas which has been 
_ absorbed by the leaf of the tree from the atmosphere, is there 
*€ dissociated,” or separated into its elements carbon and oxygen, 
_ the oxygen being returned to the atmosphere, and the carbon 
retained to form the solid substance of the tree. 
It is thus clearly shown that the sun has to’ impart 11,000,000 
units of energy to the tree for the formation of one pound of 
' carbon in the shape of woody fibre, and that these 11,000,000 
units of energy will be simply resuscitated when the wood is 
burnt, or again combined with oxygen to form carbonic acid, 
Fuel, then, is derived through solar energy acting on the 
surface of our earth. 
But what about the stores of mineral fuel, of coal, which we 
find within its folds? How did they escape the general com- 
bustion which, as we have seen, has consumed all other elemen- 
tary substances? The answer isa simple one. These deposits 
of mineral fuel are the results of primeval forests, formed in the 
-manner of to-day through the agency of solar rays, and covered 
over with earthy matter in the many inundations and convulsions 
of the globe’s surface, which must have followed the early 
solidification of its surface. Thus our deposits of coal may be 
looked upon as the accumulation of potential energy derived 
- directly from the sun in former ages, or as George Stephenson, 
with a sagacity of mind in advance of the science of his day, 
answered, when asked what was the ultimate cause of motion of 
his locomotive engine, ‘‘that it went by the bottled-up rays of 
the sun.” 
It follows from these considerations that the amount of poten- 
tial energy available for our use is confined to our deposits of 
coal, which, as appears from the exhaustive inquiries lately 
made by the Royal Coal Commission are still large indeed, 
but by no means inexhaustible, if we bear in mind that our re- 
quirement will be ever on the increase and that the getting 
of the coal will become from year to year more difficult as we 
descend to greater depth. To these stores must be reckoned 
lignite and peat, which, although not coal, are nevertheless the 
result of aie energy, attributable to a period of the earth’s 
creation subsequent to the formation of the coal beds, but an- 
terior to our own days. 
In discussing the necessity of using our stores of fuel more 
economically, I have been met by the observation that we need 
not be anxious about leaving fuel for our descendants—that the 
human mind. would surely invent some other sourcé of power 
when coal should be exhausted, and that such a source would 
probably be discovered in electricity. 1 heard such a suggestion 
publicly made only a few weeks back at a meeting of the Inter- 
national Jury at Vienna, and could not refrain from calling at- 
tention to the fact that electricity is only another form of energy, 
that could no more be created by man than heat could, and 
involved the same recourse to our accumulated stores. 
If our stores of coal were to ebb, we should have recourse, no 
doubt, to the force radiating from the sun from day to day ; and 
it may be as well for us to consider, what is the extent of that 
force, and what our means of gathering and applying it. We 
have, then, in the first place, the accumulation of solar energy 
upon our earth’s surface by the decomposition of carbonic acid 
in plants, a source which we know by experience suffices for the 
NATURE 
443 
human requirements in thinly-populated countries, where in- 
dustry has taken only a slight development. Wherever popula- 
tion accumulates, however, the wood of the forest no longer 
suffices even for domestic requirements, and mineral fuel has 
to be transported from great distances. 
The sun’s rays produce, however, other effects besides vege- 
tation, and amongst these, evaporation is the most important 
as a source of available power. By the solar rays, an amount of 
heat is imparted to our earth that would evaporate yearly a lake 
of water fourteen feet deep. A considerable proportion of this 
heat is actually expended in evaporating sea water, producing 
steam or vapour, which falls back upon the entire surface of both 
land and sea in the form of rain. The portion which falls upon 
the elevated land flows back towards the sea in the form of 
rivers, and in its descent its weight may be utilised to give 
motion to machinery. Water power, therefore, is also the re- 
sult of solar energy, and an elevated lake may indeed be looked 
upon as fuel, in the sense of its being a weight lifted above the 
sea level through its prior expansion into steam. 
This source of power has also been largely resorted to, and 
might be utilised to a still greater extent in mountainous coun- 
tries; but it naturally so happens that the great centres of 
industry are in the plains, where the means of transport are 
easy, and the total amount of available water-power in such 
districts is extremely limited. 
Another result of solar energy are the winds, which have been 
utilised for the production of power. This source of power is, 
indeed, very great in the aggregate, but its application is at- 
tended with very great inconvenience. It is proverbial that 
there is nothing more uncertain than the wind, and when we 
were dependeat upon windmills for the production of flour, 
it often happened that whole districts were without that neces- 
sary element to our daily existence. Ships also, relying upon 
the wind for their propulsion through the sea, are often becalmed 
for weeks, and so gradually give preference to steam-power on 
account of its greater certainty. It has been suggested of late 
years to utilise the heat of the sun by the accumulation of its 
rays into a focus by means of gigantic lenses, and to establish 
steam-boilers in such foci. This would be a most direct utilisa- 
tion of solar energy, but it is a plan which would hardly recom- 
mend itself in this country, where the sun is but rarely seen, and 
which even in a country like Spain would hardly be productive 
of useful, practical results. 
There is one more natural source of energy available for our 
uses, which is rather cosmical than solar, viz., the tidal wave. 
This might also be utilised to very considerable extent in an 
island country facing the Atlantic seas, like this, but its utilisation 
on a large scale is connected with great practical difficulty and 
expenditure, on account of the enormous area of tidal basin that 
would have to be constructed. 
In passing in review these various sources of energy which are 
still available to us, after we have run through our accumulated 
capital of potential energy in the shape of coal, it will have 
struck you that none of them would at all supply the place of 
our willing and ever-ready slave, the steam-engine ; nor would 
they be applicable to our purposes of locomotion, although means 
might possibly be invented of storing and carrying potential 
energy in other forms. But it is not force alone that we require, 
but heat for smelting our iron and other metals, and the accom- 
plishment of other chemical purposes. We also need a large 
supply for our domestic purposes. It is true that with an abun- 
dant supply of mechanical force we could manufacture heat, and 
thus actually accomplish all our purposes of smelting, cooking, 
and heating, without the use of any combustible matter; but 
such conversion would be attended with so much difficulty and 
expenditure, that one cannot conceive human prosperity under 
such laborious and artificial conditions. : 
We come now to the question—How should fuel be used, and T 
propose to illustrate this by three examples which are typical of 
the three great branches of consumption. 
a. The production of steam power, 
4. The domestic hearth. 
c. The metallurgical furnace. 
I haye represented on a diagram two steam cylinders of the 
same internal dimensions, the one being what is called a high- 
pressure steam cylinder, provided with the ordinary slide-valve 
for the admission and discharge of steam into the atmosphere, 
and the other so arranged as to work expansively (being provided 
with the Carless variable expansion gear) and working in con- 
nection with a condenser, Ihave also shown two -diagrams of 
