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POPULAR SCIENCE REVIEW. 
gas or oily products (tar) are produced, there remains in the 
retort a fragment of coke. 
All the products of this splitting up of coal by the action of 
heat have been largely utilized ; they have given rise to ex- 
tensive and most important branches of industry and promise 
to yield still further benefits to mankind. In former years 
coal was looked upon as a species of carbon ; but it was soon 
discovered to be a substance rather more complex in its nature 
than had been imagined: — 1001b. of coal contain on an 
average 80 1b. of carbon, 5 or 61b. of hydrogen, 21b. of 
nitrogen, 5 to 12 lb. of oxygen, \ to 1 lb. of sulphur, and 5 
to 10 lb. of mineral substances which constitute the ash ; 
it leaves in a retort after distillation about 60 per cent, of 
coke. 
Any person who has devoted a little attention to organic 
chemistry will see at once what a vast number of products may 
be formed by the grouping* together of these elements in 
various ways. Now, when coal is distilled no less than fifty- 
one distinct substances are obtained from it ; that is, fifty- one 
distinct substances have hitherto been discovered in the pro- 
ducts of the distillation of coal, maybe others will shortly 
turn up. Practically speaking, these substances are not of 
equal importance. Among those which demand our more 
particular attention here are : 1st., the gas which serves to 
light our streets. This is chiefly marsh-gas, the gas which 
bubbles up from the mud of stagnant ponds, otherwise called 
protocarburetted hydrogen, C 2 H 4 ; containing also some bi- 
carburetted hydrogen, C 4 H 4 ; and carbonic oxide, CO; a little 
free hydrogen, H ; sulphuretted hydrogen, S H ; carbonic acid 
C O 2 ; acetylen, C 4 H 2 ; &c. 2nd, the oily products or tar from 
which we obtain a variety of hydrocarbons, boiling and dis- 
tilling at different temperatures, such as benzol C 12 H 6 , toluol 
C 14 H 8 , which are liquid ; naphthaline C 20 H 8 , and paraffine (the 
composition of which is uncertain) which are solid; besides 
creosote or phenol, aniline, ammonia, and several other similar 
substances. 3rd, The coke which remains in the retort. The 
latter we will dispose of at once by stating that it is exten- 
sively used as a combustible, and also as a powerful element in 
certain forms of electric batteries. Not many years ago the 
oily products or tar were likewise burnt under the retorts of 
gasworks in order to economise the coal. Yery different are 
the uses to which they are applied at the present day ! 
This is not the place to examine the best methods of pro- 
ducing and purifying gas for illumination, nor to allude to the 
enormous amount of sulphate of ammonia annually produced 
for agricultural purposes by causing this gas to pass through 
sulphuric acid on its way to the gasometers. We must fix our 
