GAS, COAL 565 



hour. Multiplying these quantities by 60, and we obtain the quantity of gas con- 

 sumed in the hour. 



A pillar, j, having a pressure gauge, and two cocks at K, one with a micrometer 

 movement, screws on to the top of the meter, and is intended for receiving burner* 

 when experimenting. The graduated rod is supported in an exactly horizontal 

 position by the pillars c and J, and screwed together by its binding screws. 



The candle socket D is screwed on to the top of c, and the mahogany slide c placed 

 on the rod, with its pointer to the scale, carrying the frame o, containing a prepared 

 paper, and covered by the cone H. 



The prepared paper is made by coating white blotting-paper with sperm, so as to 

 render it semi-transparent, leaving a small spot in the centre plain, and therefore 

 opaque. See o in the figure. 



All that now remains to render the apparatus ready for experimenting, is to put a 

 piece of candle into the socket* and consume the gas through a proper burner over 

 the meter, taking care that the centres of the candle-flame, paper, and gas flame, are in 

 one horizontal line, and adopting the precautions previously laid down. 



Unfortunately the determination of the exact point of equality of the two lights, is 

 by no means easy, even after considerable practice ; and the maximum amount of- error 

 to which even the practised operator is liable in such estimations of illuminating 

 power, cannot be set down at less than 5 per cent. It is scarcely necessary to add, 

 that all photometric experiments must be conducted in an apartment from which all 

 light from other sources is excluded, and the walls of which are rendered as absorbent 

 as possible, by being coated with a mixture of lampblack and size, or by being hung 

 with black lustreless calico. 



Analytical method of estimating the value of illuminating gas. Frankland has shown 

 that the resources of chemical analysis place in our hands a method for the deter- 

 mination of the illuminating value of gas considerably more accurate than the 

 photometric process just described, although the execution of the necessary operations 

 requires more skill, and is usually much more troublesome. As the determination of 

 the illuminating power of a sample of gas by the analytical method necessitates most 

 of the operations required for the performance of a complete analysis of coal-gas, we 

 shall here include in our description of the former process the additional details 

 necessary for the latter. 



1. Collection of the sample of gas. In all analytical operations upon gases, it is of 

 the utmost importance, that the latter should be preserved from all admixture with 

 atmospheric air. This can only be done, either by collecting the- samples of gas 

 over mercury, or by enclosing them in hermetically 

 sealed tubes. When the sample of gas is collected 

 at the place where the analysis is to be made, the 

 former plan is usually most convenient, but when 

 the sample has to be obtained from a locality at 

 some distance from the operator's laboratory, the 

 latter plan is usually adopted. To collect a sample 

 of gas over mercury, attach one end of a piece of 

 vulcanised India-rubber tube to the-gas pipe, and 

 insert into the other extremity a piece of glass tube c | 

 bent, as shown at A, fig. 1020, allow the gas to 

 stream through these tubes for two or three minutes, 

 and then suddenly plunge the open extremity of the glass tube beneath the surface of 

 the mercury in the trough c. Then fill the small gas jar B completely with mercury, 

 taking care to remove all air-bubbles from its sides by means of a piece of iron wire, 

 and closing its mouth firmly with the thumb, invert it in the trough c, introducing 

 the end of the bent tube -A into its open extremity, in such a way as to bring the mouth 

 of A above the level of the surface of the mercury in c. The gas will then flow into B, 

 until the level of the mercury in B is somewhat lower than that of the metal in the 

 trough. If now, the tube A being removed, a small cup be filled with mercury and 

 brought beneath B, the latter may be removed from the trough, and will be thus pre- 

 served from any appreciable atmospheric intermixture for several months. 



To collect samples of gas in hermetically- sealed tubes, proceed as follows : take a 

 piece of glass tube about f ths of an inch internal diameter, and 1 foot long ; draw it 

 out at both ends before the blowpipe, as shown in fig. 1021 ; attach one extremity, A 

 fig. 1022, to a vulcanised India-rubber tube, communicating with a source of the gas, 

 and the opposite extremit y B, to a similar flexible tube about three feet long, and which 

 is allowed to hang down perpendicularly from B. After the gas has streamed through 

 this system of tubes for about three minutes, so as to ensure the complete expulsion of 

 atmospheric air, the flame of a mouth blowpipe is directed against the narrow portion 

 of the glass tube, at c, so as to fuse it off. With as much expedition as possible, the 



