Experiments on the different Mnds of Coal. 293 



very generally is. The more capable the coal is of swelling 

 (the third class), the more does the proportion of oil gas increase 

 in the gaseous mixture. 



It is only in those coals of the first and second classes in which 

 the quantity of charcoal is small, that a decomposition of the 

 combustible is effected before it has experienced a red heat ; 

 and even in these coals the decomposition does not make a 

 marked progress at a low temperature. 



The oily substance never begins to be developed until the 

 heat has attained the degree of deep red. To all the coals of 

 the two first classes, as well as those of the third, which contain 

 inuch carbon, a low red heat must be applied to begin the de- 

 composition, and a very strong red heat to terminate it. All 

 the varieties of coal, besides oil and gas, also disengage water, 

 on being distilled in the dry way. 



In the ordinary trials of coals, the object of which is to deter- 

 mine the quantity and kind of coke or charcoal which they are 

 capable of furnishing by dry distillation, the coals are usually 

 employed in a state of desiccation in the air. This method is 

 sufficient for common purposes ; but it does not answer for che- 

 mical analysis, properly so called . In this latter case, M. Kar- 

 sten found it necessary to dry, at the temperature of boihng 

 water, the various combustibles which he intended to analyse 

 chemically, with the view of comparing the results of the analy- 

 sis, with the effects which the same coals produce on being sub- 

 mitted to dry distillation. 



The author had at first presumed, that all varieties of coal, 

 taken in their ordinary state of desiccation in the air, and such 

 as they are employed for carbonisation, would not undergo a 

 great loss of weight at any temperature below that of boiling 

 water ; or that, at least, this loss of weight would ,be nearly 

 equal in all. But in order to attain his object, he found him- 

 self obliged to enquire what loss of weight coals experience 

 from desiccation, at the temperature of boiling water. Hence 

 a series of comparative trials which M. Karsten also extended to 

 some other substances. 



All these matters reduced to powder, were first exposed dur- 

 ing five days, under the same circumstances, to a temperature of 

 from 11 to 12 degrees of Reaumur's thermometer. When they 



JANUARY MARCH 1827. TJ 



