378 
Petroleum : its Sources and Uses. 
and already the records of the Royal Agricultural Society show 
that several successful forms of motor are in the field, the tendency 
being to simplify tlie details and to render them less delicate in 
adjustment. But much still remains to be done. The useful work 
on the brake is under 1 4 per cent, of the energy latent in the fuel ; 
while the heat carried off by the water-jacket round the cylinder and 
by the exhaust is equivalent to 75 per cent, of the total thermal 
capacity of the oil. This loss surely constitutes a storehouse from 
which we may hope to appropriate a good deal. I think that 
probably a combination of the direct combustion engine with the 
spirit engine of the Yarrow type will give the best results, espe- 
cially if a more advantageous cycle than that of the Otto gas 
engine can be adopted. 
As a lubricant also petroleum is taking a prominent place. The 
circumstance that it is devoid of fatty acids makes it peculiarly 
fitted for use with steam machinery, and for work which it is desired 
to protect from rust or verdigris. It can be obtained also of any 
degree of fluidity, from the most mobile of liquids to the consistency 
of jelly, while its cheapness serves to recommend it to every 
consumer. 
There are probably few people who, having realised the rapid 
increase in the use of petroleum, have not asked themselves the 
question, whether the stores of petroleum in the bowels of the earth 
will long be able to stand the demand made upon them. I will not 
enter into statistics, because, when we come to talk of fifty or a 
hundred millions of barrels being annually consumed, the figures do 
not coirvey any clear idea, at any rate to my own mind, of the 
magnitude of the consumption ; it is however already very great, and 
is increasing with extraordinary rapidity, doubling in about ten years. 
The statistical trade returns, besides, take no account of the enor- 
mous volume of natural gas evolved in some localities, nor of the 
waste which occurs when the fountains of petroleum get out of 
control. 
It is commonly assumed, without any good reason however, that 
petroleum is of the nature of coal, and has been formed, like it, out 
of the debris of primeval forests or out of the remains of marine 
animals ; and that, like coal, the deposit will be exhausted in time. 
But it seems not unlikely, as the distinguished Russian chemist Dr. 
Mendeleeff has suggested, that petroleum is constantly being formed 
by the action of water on metallic deposits in the lieated interior of 
the earth ; and that there is good hope, therefore, not only that rock 
oil can never be exhausted, but that it will be found in most parts 
of the earth if borings sufficiently deep be made. It should be 
borne in mind, moreover, that the deptli of a boring adds very little 
to the cost of getting, because the oil usually rises naturally to the 
surface or very nearly to it. 
Petroleum is an almost pure hydro-carbon, the American variety 
having a composition homologous with marsh gas or fire-damp, CH 4 
(where C denotes carbon, and H hydrogen) ; that is to say, composed 
according to the general formula C„H 2 „+ 2 > the value of n ranging 
