ON THE DEVELOPMENT OF LIGHT FROM COAL-GAS. 245 



the illuminating power is improved, and some experiments made in the 

 laboratory of the University of Llunich go to show that an increase of 

 18 per cent, in the illaminating power -was produced by heating the gas 

 from 64^ degrees to 288 degrees Fahrenheit. The London Gas Referees, 

 in an able report on the construction of gas-burners, issued in 1871, 

 repeated this experiment, and found no appreciable difference in the 

 illuminating power of gas on heating the gas before burning from about 

 68 degrees to 296 degrees Fahrenheit. One of us has recently tried the 

 same experiment. The gas was caused to pass through about 6 feet of 

 copper tubing, heated to dull redness. By this means the gas was heated 

 from 58 degrees up to 350 degrees, as indicated by a thermometer placed 

 in the current of the gas within 6 inches of the burner. It was found 

 necessary to open wider the tap of the meter as the temperature rose, in 

 order to pass exactly the required quantity of 5 cubic feet per horn*, the 

 heated and expanded gas requiring more time to pass through the burner 

 than the same quantity of cold gas. Careful observations were made of 

 the illuminating power as the temperature rose. The result was that no 

 appi-eciable difference could be seen in the illuminating power even at the 

 highest temperature reached — 350 degi-ees Fahrenheit — thus confirming 

 the results obtained by the London Gas Referees. As the temperature of 

 combustion would be increased by heating the gas, and consequently a 

 higher degree of incandescence produced, some increase of the illumi- 

 nating power may be expected, but the increase of temperature tried 

 (and it is very difficult to heat the gas even so high as 350 degrees) is 

 evidently too insignificant to produce any appi-eciable increase in the illu- 

 minating power. 



An experiment to try the effect of heating the air supplied to the 

 burner was more successful in producing an appreciable impi-ovement in 

 the illuminating power. The air was supplied from a holder under 

 pressure. It was passed through a heated copper tube, and from thence 

 into the bottom of the standard Argand burner, which was closed, 

 excepting to the admission of the heated air. A thermometer was fixed 

 in the current of heated air about 6 inches from the burner. There was 

 no difficulty in heating the air to a temperature of 520 degrees Fahrenheit. 

 At this heat the soldering of the apparatus gave Avay, so that no higher 

 temperature was tried. The temperature of the uuheated air was 70 

 degrees, and the gas used, when supplied with air of this temperature, 

 gave a light equal to 16 candles per o cubic feet per hour. As the tem- 

 perature of the air was increased, the illuminating power gradually rose, 

 until at 520 degrees a light equal to 17'5 candles was pi-oduced — a rise of 

 a candle and a half, or about 9 per cent., for an increase of 450 degrees in 

 the temperature of the air supply. As the amount of heat sujjplied by 

 the heated air brought into contact with the gas and the flame is consider, 

 able, an appreciable effect is produced on the temperature of the flame, 

 and consequently on its illuminating power. It would appear, however, 

 that the principle of heating the air supply is not likely to be a lopted for 

 general lighting purposes, for the additional light which any practicable 

 amount of heating would cause to be obtained would probably not com- 

 pensate for the exti-a cost and trouble attending the use of the required 

 apparatus. 



A number of burners of various kinds, now supplied to the public, 

 have been tested with common coal gas, having an illuminating power 



