Different Kinds of Coal for the purpose of Illumination. 1 59 



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warranted in concluding, that the pressure corresponds with the 

 specific gravity, and that consequently the specific gravity may 

 be ascertained by the pressure, when the gases are consumed un- 

 der similar circumstances ; having of course previously determin- 

 ed the specific gravity of a gas known to require a certain pres- 

 sure for its consumption. Consequently the specific gravities 

 being ascertained experimentally, the consumptions may be 

 known ; these being, the times for equal consumptions, as the 

 roots of the specific gravities, and the consumptions in equal times ,. 

 inversely as these roots. 



TX m 



Having established these positions, the next object I had in 

 view was to ascertain, by a more extended series of trials, whether 

 an indication of the consumption of gases, in different places, 

 could thus be obtained, without having recourse to a metre, which, 

 to an experimenter, carrying one from place to place, is trouble- 

 some. It at first occurred to me that this might be done by as- 

 certaining the specific gravity ; but this also must be a trouble- 

 some process, and unless ascertained with accuracy, would not 

 tend to accurate results, especially in a hurried visit to different 

 gas-works, where the height of the barometer cannot always be 

 observed. After numerous trials, I at last determined to have re- 

 course to the pressure-gauge, consuming the gas with a given 

 height of flame, and always from the same burner. For this 

 purpose, I used in my first trials the platinum-jet formerly alluded 

 t0 > with an aperture of ft of an inch, furnished with a scale for 

 measuring the height of flame, and with a pressure-gauge, to 

 which a graduated scale, with a vernier, was applied, and by 

 which the pressure at the burner, necessary to keep up the com- 

 bustion at the desired height of flame, could be measured to 

 lOOths of an inch. The diameter of the pressure-gauge was 

 half-an-inch, which, though it gave a considerable curve in the 

 fluid, yet afforded more accurate results than when one with a 

 smaller diameter was used. Indeed, I had recourse to various 

 contrivances, such as floats in the water, with indices attached to 

 them, and made to move on the scale ; but I found, after numer- 

 ous trials, that the simple wide gauge was by far the best, provi- 

 ded that care was taken to mark the height of the fluid from the 

 same part of the curve. I always marked it from the bottom : 

 a ud for this purpose had brass plates passing round the tube, and 

 Movable, so that they could be brought to the curve in both limbs. 

 In the following set of experiments, the gases were prepared 

 from a variety of coals by means of the experimental apparatus. 

 They were always consumed with a five-inch flame. The illu- 

 minating power was tried by the chlorine test. The specific 

 gravity was taken in the usual way, by filling a vessel of known 

 capacity with the gas, and noting at the same time the state of 

 the thermometer and 'barometer. Not less than two trials were 



