25 MARCH 1876, Page 13

TESTING GASLIGHT.

(To THa EDIT01 OF THE 13 ram ATO SIR,—In a paragraph of the Spectator last week you referred to 'the application of Dr. Crookea's radiometer to the measurement of light, to illustrate, in your opinion, that the London gas is not fairly tested by the present photometrical appliances. The statement made originally by that gentleman was, I believe, that by balancing two sources of light one against the other, the -standard candle, 48 inches off, balances the pith-bar at zero with a " small" gas-burner at 113 inches, viz.,- 482 : 1132 : : 1 : St candles.

Now in the process of photometry, as it is now practised, the expression "standard candle" means a sperm candle burning ab- solutely 120 grains per hour, and a "burner" means an "Argand" burner consuming five cubic feet per hour. En dehors of those conditions, the comparison, as a test, would be Utterly devoid of -value, and it is the universal practice of all gas examiners to reject any test which does not as nearly as possible conform to these vales. The expression of " small " burner, therefore, obviously qualifies the result.

That London gas has been within a few years raised theoretically in illuminating power, and not practically, is quite true, but any determination of the radiometer itself (assuming it to be a correct measurer) would raise it theoretically quite as much, because it is the gas-burner through which the learned professor is obliged to measure the light comparatively with the standard candle which regulates within a margin of several candles, the actual quality of gas recorded. I. believe it was Dr. Letheby who, explaining the principles of photometry before a meeting of the Associa- tion of Medical Officers, stated that gaslight cannot be improved "by any burner, all burners, from the best downwards, acting upon one common principle—that of destroying light—and the best burner will consequently ever be that one which de- stroys it the least. Sixteen years ago it was believed that an "Argand" burner with fifteen holes, used with a seven- inch chimney, was the best form of burner to consume gas in, and as it was an accepted fact that a ton of ordinary Newcastle coals will not yield more than 9,500 cubic feet of such twelve- candle gas (any excess in quantity over this amount reducing the illuminating power to an alarming extent), it was stipu- lated that the gas to be supplied to this Metropolis should 'be twelve-candle gas, consumed at the rate of five cubic feet per hour in a burner of that kind. When, in 1868, the Corpora- tion of London promoted a Bill dealing with the Companies within its own precincts, that standard was raised to sixteen candles ; and later, the Imperial and the South-Metropolitan Companies were, 'by their Acts of 1869, both raised to fourteen candles. The Com- mercial, last year, with the Ratcliff, adopted sixteen candles, so that at present, under the nominal standard of twelve candles, there remain but the Phoenix, the London, the Surrey Consumers, and the Independent Companies. I say "nominal," because they practically all give sixteen candles, tested with the present new and improved standard burners now prescribed for testing by the gas 'referees; and it is an admitted fact that, although the " burner " has been improved, the "light has not," unless the public take the trouble to purchase and use the beat form of burners obtainable. It is clearly, however, the opinion of the gas referees that this duty devolves upon the public. I do not offer any opinion on that ,point.

But referring back again to the mode of estimating this illu- minating power, it is clear to me that the radiometer in nowise differs from the old Bunsen photometer in principle so long as it estimates by comparison light issuing from a gas-burner ; and if the comparison is made in any other way, by which the medium of the burner is dispensed with, or the light measured according to its literal value, then, I say, the test must prove manifestly un- fair to the consumer, who only measures its reduced value through -the medium of a burner. If in a Bunsen photometer, with the candle at ten inches from the disc—the burner at twenty-four inches—the shadows on the disc are equal, we draw this conclu- sion, under the well-known law that light decreases in intensity in the ratio of the square of its distance off the photometer-disc :-

10= : 24' : 1 : 5.76 candles.

This is a law which is not likely to be affected by any discovery, and I venture to think that Dr. Crookes is entitled to take greater credit for this really beautiful discovery of the radiometer, upon the evidence of its apparent concordance with the general results of our photometrical knowledge, than in any other respect.— 1 Westminster Chambers, S.W., March 21.