英文翻译

Determination of the Illuminating Power of Chal-gas.—The value of coal-gas naturally depends chiefly on its .illuminating power. This can be determined in various ways. Of these， the most practically useful and easiest is the photometri... Determination of the Illuminating Power of Chal-gas.—The value of coal-gas naturally depends chiefly on its .illuminating power. This can be determined in various ways. Of these， the most practically useful and easiest is the photometric method， the amount of light emitted by a gas flame， burning at the rate of five cubic feet per hour， being compared with that given off by a standard sperm candle consuming 120 grains of sperm (7*79 grams) per hour. For this purpose， Btmsen's photometer， see Fig. 211， is employed. This depends upon the fact that the intensity of the illumination from a luminous point is inversely proportional to the square of the distance of the illuminated surface from that point. For flames- of coal-gas or candles， this law is of course only approximately true， but sufficiently so for practical purposes， and it， therefore， serves as the basis of a useful， though not very accurate， method of testing the illuminating power of coal-gas. All that is neces-* savy is to ascertain the distances at which the standard candle and the gas-flame both produce the same illuminating effect; it then only remains to take the squares of these distances in the inverse order to obtain the relative illuminating power of the two sources of the light. In Bunsen's photometer， the surface illuminated by the candle- and gas-flames， consists of a diaphragm of paper， Fig. 211， which， with the exception of a small circle in. the centre， Fig. 212， has been painted over with a solution of spermaceti in benzene， which renders the whole surface， with the exception of the central disc， transparent. This diaphragm is placed in a frame fixed to a moveable slide， which fits on to the long divided tod of the photometer (o'o)， Fig. 211. Over this frame is placed a black shade containing two pieces of mirror-plate ao fixed， that when the observer looks at the diaphragm in a direction at right angles to the divided rod， he sees in the two mirrors a reflection of each side of the waxed paper At the right hand extremity of the rod is fixed the standard candle， whilst at the other the gas flame is allowed to burn， the rate of its combustion being accurately determined by means of the experimental gas-meter (p).