24 



almost exclusively employed as an artificial manure. Coal-tar, 

 which is an extremely complex mixture, affords the materials for the 

 production of a large number of very beautiful dyes, popularly known 

 as the aniline dyes, although the majority have little or no con- 

 nexion with aniline. But few of the constituents of coal-tar, 

 however, are at present utilized, and there is abundant opportunity 

 for the discovery of new applications. Only quite recently a method 

 of manufacturing indigo — one of the most important dye-stuffs 

 liiiown — from a constituent of coal-tar, has been elaborated and 

 patented, and it is scarcely too much to expect that within very few 

 years natural indigo will be to a large extent displaced by the 

 artificial product, a result of no mean impoi-tance, as the annual 

 value of the indigo produced in various parts of the world is over 

 £3,000,000. 



Coal gas consists chiefly of hydrogen and methane or marah gas, 

 in the proporiion of about 50 of the former to 3fi of the latter. In 

 addition to these, it contains small quantities of nitrogen, carbonic 

 oxide, and of certain hydrocarbons, i.e.. compounds of carbon and 

 hydrogen. Hydrogen and methane bura with a non-luminous flame, 

 and the illuminating power of coal gas is entirely conditioned by the 

 presence in it of relatively very small amounts of hydrocarbons, such 

 as acetylene, ethylene, benzolene and naphthalene. 



The light furnished by coal gas of a given quality varies 

 greatly, however, according to the manner in which it is burnt, the 

 temperatui'e of the air supplied to it. and to some extent also with the 

 atmospheric pressure. Regarding the influence of atmospheric 

 pressure, reference was made to some experiments of Pi"of. Frankland 

 which showed that a faU in the barometer from 30'2 to 28-2 

 inches reduced the amount of light emitted by a given flame from 

 100 to 91 "4, a fui-ther diminution in pressure of two inches 

 reducing it to 80 '6. 



A veiy considerable increase in illuminating power is effected 

 by heating the air supplied to the flame. Dr. Armstrong described a 

 modification of the ordinary argand burner so arranged that this could 

 be effected, the result being that instead of a light equal to that 

 of 13 candles — the light obtained when the gas was burnt in the 



