80 
THE NATURE AND 
[sect. II. 
114. There yet remains a substance which seems to possess the properties 
desirable in the acting vapour of an engine. It is called oil gas vapour, and is 
separated from oil gas by the compression used to render that gas portable. It 
has been examined by Mr. Faraday, 1 who found that it is insoluble in water 
except in very minute quantities. It boils at about 170°, but remains liquid at 
common temperatures : it consists of a combination of fluids of different degrees 
of volatility, and by repeated distillations at different temperatures the volatile 
fluids may be separated; the most abundant separates between 170° and 200°. 
At common temperatures the fluid which separates between 170° and 200° 
appears as a colourless transparent liquid, of the specific gravity 085 at 60°, having 
the general odour of oil gas. Below 42° it is a solid body, which contracts much 
during its congelation. At zero it appears as a white or transparent substance, 
brittle, pulverulent, and of the hardness nearly of loaf sugar. It evaporates entirely 
in the air, and when its temperature is raised to 186°, it boils, furnishing a vapour, 
which is 2*7 times the weight of the same bulk of common air. It appears, how 
ever, that at a higher temperature the vapour is decomposed, depositing carbon. 
It is composed of six volumes of carbon, and three volumes of hydrogen, con 
densed into one. 
115. In a paper in the ‘Philosophical Transactions’ on the application of 
liquids formed by the condensation of gases as mechanical agents, Sir H. Davy 
anticipates the probability of the application of the elastic force of compressed gases 
to the movement of machines. 2 He founds this anticipation upon the immense 
difference between the increase of elastic force in gases under high and low tem 
peratures by similar increments of temperature. The force of carbonic acid was 
found to be equal to that of air compressed to At at 12°, and of air compressed 
to -h at 32°, making an increase of pressure equal to the weight of 13 
atmospheres. 
116. I think, however, it will be found, that two other circumstances should 
be considered in estimating the fitness of compressed gases as mechanical agents. 
First, The distance through which the force will act; for if this distance of its 
action be less in the same proportion, as the force is increased by compression, no 
advantage will be gained ; the power of a mechanical agent being jointly as the 
force, and the distance through which that force acts. Secondly, The quantity of 
heat required to produce the change of temperature is also to be considered. For 
if the mechanical power requires as great an expenditure of heat as common 
steam, no advantage worthy of notice would be gained. In fact the only prospect 
they afford of being useful, is through lessening the extent of surface to be heated. 
1 Philosophical Transactions, 1826. 
2 Idem, for 1823.