80 THE NATURE AND [SECT. n. 



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, * 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 0'85 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. * He founds this anticipation upon the immense 

 difference between the increase of elastic force in gases uiider 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 ^V at 12, and of air compressed 

 to -$& 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 w r ould 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. = Idem, for 1823. 



