544 GAS, COAL 



composition known to bo present in coal-gas is naphthaline, C 20 IP (C^H 8 ), which, 

 although a solid at ordinary temperatures, yet emits a considerable quantity of vapour ; 

 in fact, its presence occasions to a great extent the peculiar odour of coal-gas. 



Naphthaline is a frequent source of serums annoyance to the gas-manufacturer, by 

 condensing in the street mains and gradually blocking them up, or so narrowing their 

 bore as to prevent the passage of the needfdl supply of gas. This effect can only be 

 produced when the gas charged with naphthaline- vapour is allowed to leave the holder 

 at a temperature higher than that of the mains through which it subsequently flows ; 

 but as this cannot always bo avoided, the prevention of such deposits might perhaps 

 be best effected by passing the gas over a largo surface of coal-oil before it is led into 

 the mains. The oil would absorb so much of the naphthaline as to prevent any sub- 

 sequent deposition. The vapour of naphthaline contains, in an equal volume, live 

 times as much carbon as defiant gas. The amount of light yielded by these illumi- 

 nating constituents is directly proportionate to the amount of carbon contained in an 

 equal volume of each ; taking, therefore, the illuminating power of olofiant gas as 

 unity, the following numbers exhibit the relative illuminating values of equal volumes 

 of the several luminiforor.s constituents of gas : 



Propylono 

 Butyl< 



one 



Amylene 

 Hydride of amyl . 

 Hydride of ho.xyl. 

 Hexylene . 



1-5 

 2-0 

 2-5 

 2-5 

 3-0 

 3-0 



Benzol . . 3'0 



Toluol . . 3-5 



Heptylene . . 3 '5 



Cumol . . 4-0 



Cymol . . 5'0 



Naphthaline . . 6*0 



II. DILUENTS. 



Hydrogen. This element constitutes one ninth of the total weight of the waters of 

 our globe, and with one or two unimportant exceptions, enters into the composition of 

 all animal and vegetable substances and of the products derived from them, as peat, 

 coal, oils, bitumen, &c. It is, however, very rarely met with in nature in a free or 

 uncombined state ; having hitherto only been thus' found in the gases emitted from 

 volcanoes. 



Hydrogen gas may be obtained in abundance and nearly pure by passing steam 

 over iron, zinc, and several other metals, in a fine state of division, at a full red heat. 

 Mixed with carbonic oxide and carbonic acid gases, it is also generated in largo 

 quantity when steam is passed over charcoal, coke, or other carbonaceous substances 

 at a red heat. In all these cases the watery vapour is decomposed, its hydrogen being 

 liberated, whilst its oxygen unites with the metal or carbon, forming in the first case 

 a solid non-volatile oxide, which encrusts the pure metal, and soon stops further action ; 

 in the second case a gaseous oxide of carbon is generated, and passes off along with 

 the hydrogen, thus leaving the carbon freely exposed to the further action of the 

 watery vapour. When carbon is used, that portion of the steam which is converted 

 into hydrogen and carbonic oxide yields its own volume of each of these gases ; and 

 that portion which forms hydrogen and carbonic acid affords its own volume of hy- 

 drogen and half its own volume of carbonic acid. The amount of watery vapour 

 which undergoes the latter decomposition decreases as the temperature at which the 

 operation is conducted increases. At a white heat scarcely a trace of carbonic acid is 

 produced. 



Hydrogen is the lightest of all known bodies, its specific gravity being only '0691 : 

 100 cubic inches, at 60 Fahr. and 30 inches barometric pressure, weigh only 2 - 1371 

 grains. It has a powerful affinity for oxygen, but develops scarcely any light during 

 combustion ; when, however, solid substances, such as lime, magnesia, or platinum, are 

 held in the flame of hydrogen, considerable light is emitted. Burnt in air or oxygen 

 gas, it is entirely converted into watery vapour, which condenses upon cold surfaces 

 held above the flame. 



Light carburetted Hydrogen. This gas consists of carbon and hydrogen in the propor- 

 tion of 6 parts by weight of the former clement combined with 2 parts of the latter. 

 Owing to its being copiously generated in marshy swampy places, it is frequently 

 termed marsh gas, and from certain considerations relative to its chemical constitution, 

 it has more recently received the name of hydride of methyl. It enters largely into 

 the composition of coal-gas, and is also a natural product of the slow decomposition of 

 coal, and of putrefaction in general. Thus it occurs in enormous quantities in the 

 coal strata, and bubbles up from stagnant pools and ditches which contain putrefying 

 organic remains. As thus generated, it is mixed with small quantities of carbonic ai-id 

 and nitrogen ; it can, however, be artificially prepared perfectly pure, but the processes 

 need not be described here, 



