COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN 401 



But aqueous vapour (and only when strongly superheated, otherwise 

 it cools the charcoal) only acts on charcoal to form a large amount of 

 carbonic oxide at a very high temperature (at which carbonic anhydride 

 dissociates) ; it begins to react at about 500, forming carbonic 

 anhydride according to the equation C + 2H 2 O = CO 2 + 2H 2 . Besides 

 this, carbonic oxide on splitting up forms carbonic anhydride, and 

 therefore water gas always contains a mixture 26 in which hydrogen 

 predominates, the volume of carbonic oxide being comparatively less, 



26 Water gas, obtained from steam and charcoal at a white heat, contains about 50 p.c. 

 of hydrogen, about 40 p.c. of carbonic oxide, about 5 ptc. of carbonic anhydride, the 

 remainder being nitrogen from the charcoal and air. Compared with producer gas, which 

 contains much nitrogen, this is a gas much richer in combustible matter, and therefore 

 capable of giving high temperatures, and is for this reason of the greatest utility. If car- 

 bonic anhydride could be as readily obtained in as pure a state as water, then CO might be 

 prepared directly from CO 2 + C, and in that case the utilisation of the heat of the carbon 

 would be the same as in water gas, because CO evolves as much heat as H 2 , and even more 

 if the temperature of the smoke be over 100 J , and the water remains in the form of vapour 

 (Note 25). But producer gas contains a large proportion of nitrogen, so that its effective 

 temperature is below that given by water gas ; therefore in places where a particularly 

 high temperature is required (for instance, for lighting by means of incandescent lime or 

 magnesia, or for steel melting, &c.), and where the gas can be easily distributed through 

 pipes, water gas is at present held in high estimation, but when (in ordinary furnaces, 

 re-heating, glass-melting, and other furnaces) a very high temperature is not required, 

 and there is no need to convey the gas in pipes, producer gas is generally preferred on 

 account of the simplicity of its preparation, especially as for water gas such a high 

 temperature is required that the plant soon becomes damaged. 



There are numerous systems for making water gas, but the American patent.of T. Lowe 

 is generally used. The gas is prepared in a cylindrical generator, into which hot air is 

 introduced, in order to raise the coke in it to a white heat. The products of combustion 

 containing carbonic oxide are utilised for superheating steam, which is then passed 

 over the white hot coke. Water gas, or a mixture of hydrogen and carbonic oxide, is 

 thus obtained. 



Water gas i sometimes called ' the fuel of the future,' because it is applicable to all 

 purposes, develops a high temperature, and is therefore available, not only for domestic 

 and industrial uses, but also for gas-motors and for lighting. For the latter purpose 

 platinum, lime, magnesia, zirconia, and similar substances (as in the Drummond light, 

 Chapter III.), are rendered incandescent in the flame, or else the gas is carburetted 

 that is, mixed with the' vapours of volatile hydrocarbons (generally benzene or naphtha, 

 naphthalene, or simply naphtha gas), which communicate to the pale flam.e of carbonic 

 oxide and hydrogen a great brilliancy, owing to~the high temperature developed by\the 

 combustion of the non-luminous gases. As water gas, possessing these properties, may 

 be prepared at central works and conveyed in pipes to the consumers, and as it may be 

 produced from any kind of fuel, and ought to be much cheaper than ordinary gas, it may 

 as a matter of fact be expected that in course of time (when experience shall have deter- 

 mined the cheapest and best way to prepare it) it will not only supplant ordinary gas, but 

 will with advantage everywhere replace the ordinary forms of fuel, which in many respects 

 are inconvenient. At present its consumption-spreads principally for lighting purposes, 

 and for use in gas-engines instead of ordinary illuminating gas. In some cases Dowson 

 gas is prepared in producers. This is a mixture of water and producer gases obtained 

 by passing steam into an ordinary producer (Note 19), when the temperature of the 

 carbon has become sufficiently high for the reaction C + H-jO = CO H- H 4 . 



