FUELS AND THEIR UTILIZATION. 47 



Lately, however, only the air necessary for combustion of the gas is heated. 

 This has been found more profitable, as a long system of flues conducting gas 

 and air in parallel canals may, by occasionally occurring leaks, mix in the re- 

 generators, cause melting, and thereby disturb the process of manufacture. 

 Besides, the volume of air necessary for combustion is about six times the 

 volume of gas to be burned, and for these as well as for other reasons it seems 

 to be of higher importance only to heat the air intended for combustion of 

 the gas. 



BRIQUETTES. 



Of higher economic importance still than the utilization of brown coal in 

 the form of coke has become the manufacture of briquettes from lignites. 

 This article is very extensively and successfully used in Europe as fuel for 

 household purposes, and for production of steam it is frequently preferred to 

 common stone coal. Its form is convenient for shipment, it is more cleanly 

 in use, and its heating effect generally the same, sometimes better, than that 

 of common coal. 



Like in any other industry, the first experiments in the manufacture of 

 briquettes were very crude. Generally lignites do not furnish more than 

 from ten to fifteen per cent of coal in pieces large enough for fuel, and in 

 order to utilize the refuse the same was mixed with water, formed into coal 

 bricks, and were ready for use when air dried. Though the manufacture of 

 these bricks was soon supplanted by machines, the product was not very sat- 

 isfactory for on account of their considerable water content, frequently as 

 high as forty per cent, they could not be subjected to a higher pressure, 

 were easily decomposed on exposure to the atmosphere, and in consequence 

 crumbled to dust, finally convinced that this product would not yield the 

 expected and desired success, experiments were made to evaporate the water 

 of the lignites to give the coal a higher heating capacity, and form the dried 

 product under high pressure into coal bricks, which were then called briquettes. 



The press for the manufacture of briquettes now in use in Germany was 

 invented about 1856 by Exter, in Munich, and since its early introduction 

 has been changed but. little. Till 1860 a number of experiments were made 

 in the manufacture of briquettes, almost without success; but at that time it 

 became commonly understood that the most important point in the manu- 

 facture was the drying of the lignites, and not the pressing of the coal bricks. 

 Since then the experiments have been prosecuted with new zeal in this direc- 

 tion, and at the present, after thirty years of costly experience, it may be 

 said that a lasting success of the manufacture of briquettes is secured. It is 

 necessary to reduce the water contents of the lignites to about fifteen per 

 cent before they can be profitably subjected to the press. An addition of tar 

 is, however, very rarely necessary, for the lignites generally contain enough 



