PA&AFFItf 



505 



the combined forces of chemical and cohesive attraction. Hence a considerably higher 

 temperature than that of the mere boiling-point of the resultant hydrocarbons is 

 necessary. Here, then, at the outset wo encounter an insuperable necessity for heating 

 the vapours considerably above their boiling-points to a temperature, in fact, at 

 which some degree of further decomposition must take place, and some amount of 

 permanent gas must be formed. 



2. The boiling-points of the different volatile hydrocarbons obtainable from the 

 cannel or shale, range from about 200 to about 500 Fahr. In order to drive off 

 the latter, we are compelled to raise the temperature to about 300 above the boiling- 

 point of the former, exclusive of the excess of heat required for the primary decompo - 

 sition described under No. 1. 



3. When any kind .of closed retort or oven is used which is charged internally, 

 and the coal receives its heat from the outside, it is not merely necessary to raiso 

 the temperature of the sides of such retort to the decomposing and distillation-heat, 

 but to something above this heat, as no body can give off any heat by radiation 

 or convection to another body unless it be hotter than the body which is to re- 

 ceive the heat. As the quantity of heat communicated by radiation to a given 

 surface varies inversely with the square of the distance from it, this difference of 

 temperature requires to be increased very considerably, as we enlarge the retort 

 in any manner that increases the distance of portions of the coal from its heated 

 surface. 



4. The commercial necessity of rapid working, in order to economise original out- 

 lay upon plant, drives us to enlarged retorts acting disadvantageously under No. 3, and 

 to exaggerate the excess of temperature of the retort-surface, in order to effect rapid 

 communication of heat. 



It will be at .once understood that the combined action of these four necessities is 

 to expose the vapour that is formed to the action of the greatly-superheated retort- 

 walls, and thereby decompose it. The primary object of all improvements, beyond 

 those directed to economising labour and plant, is to overcome or diminish these 

 sources of loss and deterioration ; and, in describing the different forms of retort, we 

 shall refer to their intent accordingly. 



It will be readily seen that the first of these difficulties is insuperable, that the 

 others for the most part can only be partially overcome, and thus that the process is 

 necessarily wasteful to some extent. All crude-oil works until lately presented a 

 painful manifestation of this in the flaring jets of gas that illuminated so vividly the 

 surrounding country. But this waste gas is now utilised in supplying small towns, 

 like Bathgate and West Calder in Scotland, with a cheap illuminant ; or in generating 

 heat for the boilers and retorts after the light condensible oil has been suitably re- 

 moved from it. Young's Company recover in this way a gallon of mineral spirits in 

 every thousand cubic feet of gas generated. 



The first step made in Wales beyond using the common gas-retort was to increase 

 greatly the width and diminish the height of the D-retort, and forming thereby the 



1603 



' flat D,' now very extensively used, and by most manufacturers still regarded as prac- 

 tically the most useful form of retort. As almost every firm has its particular pattern, 



