424 Prof. Magnus on the Origin of Tar in Olefiant Gas. 



sure and to the original temperature, its volume was found to be 

 doubled. The investigation showed that it consisted almost 

 wholly of pure hydrogen, which possessed scarcely the odour of 

 tar. The olefiant gas behaved in a similar manner when it was 

 conducted very slowly through a porcelain tube heated to white- 

 ness. No tar was formed, a great quantity of carbon was, on 

 the contrary, separated, and the gas that issued from the tube 

 was composed almost wholly of pure hydrogen, free from the 

 smell of tar. 



From this it may be inferred that tar is formed only when the 

 heat is that of redness, and by the action of a white heat is 

 decomposed into carbon and hydrogen. 



A nearer acquaintance with the process of -its formation was 

 only to be obtained by the examination of the composition of the 

 tar. To obtain a quantity sufficient for analysis, the olefiant 

 gas, purified as above, was conducted through a glass tube which 

 had a diameter of 04 of an inch, and a length of which equal to 

 15 inches was maintained at a red heat. To condense the vapour 

 of tar thus produced, it was conducted, in company with the 

 decomposed gases, through a long thin glass tube whose dia- 

 meter was only 0-2 of an inch. In this manner, in the course 

 of a day, a quantity of tar sufficient for analysis was obtained, 

 about 0*3 of a gramme. 



When a non- volatile organic body is exposed to an invariable 

 temperature at which it is decomposed, during the whole time of 

 heating volatile products are formed, until a non-volatile com- 

 pound remains behind. If, however, the temperature be in- 

 creased, the latter compound will be also decomposed, and 

 another non- volatile body remains. This process is repeated 

 until at length carbon alone remains. If, therefore, the tempe- 

 rature be not completely constant, the formation of different 

 volatile products is the consequence of its various changes, and 

 such products also occur when the heated substance does not 

 possess the same temperature throughout. This is, however, 

 almost always the case, for at the sides of the containing vessel 

 the heat is greater than at the centre. Hence in dry distilla- 

 tion different products of decomposition appear simultaneously ; 

 the more so, as not only the non -volatile remains of the substance 

 are decomposed with increasing temperature, but also the vola- 

 tile portions, if they are decomposable. This is the reason why 

 the tar produced from olefiant gas is a mixture of various pro- 

 ducts of decomposition ; for it is not possible to maintain the 

 temperature of the red-hot tube, in which the tar is generated, 

 perfectly constant. Besides this, the sides of such a tube are 

 always hotter than the middle, which is cooled by the gas passing 

 through the tube. Finally, the tar-vapour formed on entering 



