tn I he Distillation oft-Tood, &c. 205 



\h^ heat be caielessly managed. If the vapour of the oilv matter 

 as it aiises be exposed to the sides of the velort elevated to a 

 high temperature, it is decomposed ; and instead of oil there are 

 thus obtained bv a violent distillation in a naked fire, scarcely any 

 products tfut acetic acid and an inflammable gas. This fact isj 

 analogous to those occurring in the ordinary process for decom- 

 posing such inflammable bodies as can be made to put on the 

 gaseous state — and we ought, in fact, to consider every process 

 of this kind, where a rapid distillation with a hot fire is used, as 

 a succession of decompositions ; the matter first produced being 

 afterwards exposed to another process of destruction. It is not 

 therefore perhaps very correct language, to say that vegetables 

 yield a great quantity of inflammable gas on distillation with a 

 naked fire : this is the produce of a second distillation, which by 

 the common mode of operating is confounded with the first. As 

 tin.; reasoning applies enually to all other similar processes, it 

 would be desirable to use a more accurate mode of describing 

 this common operation, by which we might in some important 

 instances be led to a more correct practice. Thus, for example, 

 in the common mode of distilling coal to produce the inflam- 

 mable gases, this double operation is carried on at once by the 

 application of the petroleum and na|)htha at first produced to 

 the heated iron of the retort. It is in consequence of this im- 

 perfect mode of exposing tiie fluids thus generated to a second 

 heat, that so large a portion of the petroleum is distilled un- 

 changed. }5y causing it to pass a second time in contact with 

 heated iron, vvliile in the state of vapour, it may be resolved 

 completelv iiito inflammable gas and charcoal, and the produce 

 of gas be thus considerably increased. This circumstance ex- 

 plains also the contradictory accounts given by different persons 

 of the relative products of distillation, as applied to the various 

 compound inflammables. To instance the case of camphor, 

 which, according to the mode of ma'.iaging the process, may be 

 caused to yield essential oil, or inflammable gas, or a mixture of 

 both in various jjroportions : I need scarcely point out the ad- 

 vantages so obviously to be derived from this consideration in 

 tlie oeconomical process of procuring light from pit-coal, an 

 operation at present conducted with less skill than it demands. 



I distilled a portion of this tar in such a way as to obtain in- 

 flammable air only, and took the gas in five portions. The first 

 burnt very faintly, the second rather better, the third and fourth 

 portions with a good white flame, and the fifth burnt feeble and 

 blue. No portion of it was equal in brilliancy of inflammation 

 to the gas from pit-coal. On examination, it w?8 found to con- 

 tain much carbonic oxide, by which its nature, as far as it differs 

 from the ga-i of coal, is readily understood. The cause, of this 



difference 



