CHEMICAL SCIENCE. 229 



continues, while heavy hydrocarbon vapors, mixed with the vapors of water 

 and salts of ammonia, escape, and may be condensed. 



The proportion of permanent gases formed is small in comparison with the 

 weight of the liquids produced, when the decomposition of the coal is care- 

 fully regulated. In the ordinary rapid breaking up of the composition of 

 coal by heat suddenly applied in the manufacture of illuminating gas, the 

 proportion of permanent gases is increased, but the heavy fluid hydrocarbons 

 are also formed. This mode of decomposition is evidently a mixed one, 

 partaking of the characters of a regulated distillation, while at the same 

 moment a more complete destruction of the coal is proceeding in some parts 

 of the mass. A further decomposition of the fluid products, condensed from 

 .either or both of these modes of operating, takes place when we again sub- 

 ject them to the influence of heat; and this well-known fact is the basis on 

 which improvements in the manufacture of illuminating gas have been 

 founded, a secondary destruction of vapors being effected in appropriate 

 appai-atus, heated to a high temperature. 



This character, which all the bituminous coals exhibit, of passing into car- 

 bon nearly free from vapors only when heavy fluid hydrocarbons are also 

 formed, has, in a chemical view, been the strongest fact adduced in opposi- 

 tion to the generally received opinion that the anthracites and semi-anthra- 

 cites have resulted from chemical changes of bituminous coal, through the 

 agency of the heat of igneous rocks which have disturbed their beds. The 

 heavy hydrocarbons, represented by ordinary coal-tar, are the most inde- 

 structible bodies known; and wherever anthracites exist, we should expect to 

 find near by those products of the chemical changes effected in the coal. 

 Such is the delicacy of the balance existing between the elements of the 

 heaA'y hydrocarbons, that no second distillation of them can be effected; they 

 always undergo decomposition by heat, with the separation of carbon, which, 

 under any known natural conditions, would remain to attest their previous 

 presence. 



Considerations of this kind have led me to experiment on the changes 

 which coals undergo by heat, where the influencing conditions were not the 

 same as those usually seen; and the results of extended trials demonstrate 

 that the bituminous coals may be broken up into permanent gases, vapors 

 of water, and ammoniacal salts, while carbon remains as a fixed product. If 

 we substitute, for the ordinary forms of apparatus used in decomposing coal 

 by heat suddenly applied, any modification of form which compels the gas, 

 as it forms, to escape from the more highly heated part of the mass of coal, 

 through a small opening, or, better, a small eduction pipe, the heavy hydro- 

 carbons do not form part of the products which escape. Generally the light, 

 nearly colorless oils of the benzole series, appear with the aqueous solutions 

 of the ammoniacal salts, while only an accidental quantity of carbon is de- . 

 posited in the eduction pipe. The carbon left is more than usually compact 

 and hard; and such coals as ordinarily produce much water, when they form 

 heavy hydrocarbons, afford less than half the usual amount, when thus 

 decomposed, under the influence of the constant presence of an atmosphere 

 of permanent gases. 



In following the observations at the earlier stage, it was found that the size 

 of the eduction-tube leading the gas from the hotter part of the mass of coal 

 undergoing changes, exerted a most marked effect on the composition of the 

 products. It was established as a fact, that in an ordinary coal-gas retort, 



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