MANUFACTURE OF COAL-GAS. 29 



gas is colourless, and, when perfectly pure, inodorous. It is not perceptibly 

 dissolved by water : it has a powerful affinity for oxygen, and is therefore emi- 

 nently combustible. Intense heat is developed by the combustion of hydrogen 

 in oxygen gas, but little light : the compound thus produced is water. 



We have already stated that water is composed of two volumes of H to one 

 of O. Oxygen and hydrogen, although mixed in the proper proportions for 

 combining, do not unite until flame has been applied, or an electric spark 

 passed through them. Oxygen can separate hydrogen from most substances 

 with which it has previously been combined, and in analysis is frequently 

 used for this purpose, as we shall see in the sequel. Yet, under certain circum- 

 stances, there are many substances which can separate oxygen from hydrogen. 

 Carbon at a red heat is possessed of this property. If we pass steam over 

 heated coke or charcoal, the oxygen unites with the carbon, forming carbonic 

 oxide, and free hydrogen is evolved. It is on this principle that Mr. Do- 

 novan procures his hydrogen, by the impregnation of which with naphtha 

 he proposes to form an illuminating gas. Several of the metals are also pos- 

 sessed of this property, an oxide of the metal being formed, instead of carbonic 

 oxide. 



Water is too well known to need description ; it is always produced by the 

 decomposition of coal by heat. This arises from two causes, the presence of 

 hygrometric water in the coals, and likewise from its elements forming a part 

 of their composition : the former portion is the first product which passes 

 from the retorts ; the second only comes over when the coal has been resolved 

 by the heat into its elements, and the recombination of these elements has 

 been effected. This water is condensed, and carried into the tar-well, where 

 it holds in solution many of the soluble products of the distillation. 



CARBON is well known under the form of coke, charcoal, lampblack, etc. It 

 is one of the principal constituents of all varieties of coal. A knowledge of 

 the chemical properties of carbon is of great importance to the gas manufac- 

 turer, as it is the basis of the illuminating gases. 



Let us examine its compounds. It unites with oxygen in two proportions, 

 C 2 O, and C O ; the first, Carbonic Oxide, is formed when carbon is burned 

 with a minimum of oxygen, as when we burn coke or charcoal in a close 

 vessel with a limited draught. It is a colourless and inodorous gas, rather 

 lighter than atmospheric air, having a specific gravity of 0'9727 ; is sparingly 



