Page images
PDF
EPUB

rate of burning may be effected while the candles are preliminarily burning, the burner employed, if an Argaud, being always provided with a clean chimney before every experiment.

The gas examiner may ascertain whether the candles are burning at nearly the normal rate by placing a four grain weight in the candle pan when the balance turns, and observing whether the balance turns again after the lapse of one minute. Before each testing the two candles are to be so placed that the plane of the curvature of one wick shall be perpendicular to the plane of the curvature of the other wick, and the candle balance is to be so adjusted that the axes of both flames shall lie exactly between the two plumb-lines which mark the position of the zero of the scale.

Assuming that the meter is going at the rate of 5 cubic feet per hour, then look to the candles; if they have burned down much push them up until fully an inch projects above the holders. Stop the clock by lifting the lever, and bring it to zero by pressing the button at the top. Nearly counterpoise the candles by placing shot in the candle pan of the balance, leaving the candles a trifle heavier than the counterpoise. Watch the balance until the candles and the opposing weights become equipoised, and the pointer of the balance reaches the centre of its index. Then instantly start the clock by releasing the lever; place a forty grain weight in the pan of the candle holders, and proceed to make your observations in the following manner:

"Move the slide carrying the photometer disc between the gas and candle flames until a position is reached at which the spot on the disc is nearly invisible, or both sides of the disc are equally illuminated." Observe the graduation mark on the photometer bar on which the pointer of the slide rests, and record the result. When five observations have been made, reverse the disc and mirrors by turning the arrangement through half a circle.

You make ten such observations in a little over nine minutes, then give your sole attention to the candle balance, so that when it again turns, and its pointer reaches the centre of the scale, as it did when the experiment was started, you stop the clock, and note the number of minutes and seconds, or decimals of a minute, the forty grains of sperm occupied in burning.

If the rate of consumption per candle shall not have exceeded 126 grains per hour, which is equivalent to burning the forty grains in nine and a half minutes, or nine minutes thirty seconds, or shall not have fallen short of 114 grains per hour, which is equivalent to burning forty grains in ten and a half minutes, or ten minutes thirty seconds, then calculations may be made in order to neutralize the effects of the divergence from the standard rate of 120 grains per hour; but if the rate of consumption shall have varied from the prescribed rate beyond the above-named limits, then the testing is to be rejected, and a fresh testing made.

We will now suppose that the ten consecutive readings were as follows:

7.40

7.40

7.45

7.50

7.50

7.60

7.60

7.60

7.70

7.70

10)75.45

7.545

The mean of the ten readings is therefore 7.545, which multiplied by 2 (because two candles were employed) would give 15.09 as the illuminating power of the gas; but this would have to be further corrected for the consumption of the candles if each of the latter did not burn at the exact rate of 120 grains per hour, and for the barometric pressure and temperature, as well as for the consumption per hour as indicated by the experimental meter if the latter did not burn at the exact rate of 5 cubic feet per hour.

There should be no difficulty in adjusting the rate of gas to 5 cubic feet per hour, and it should not be necessary to make a correction for this; but should it be necessary to do so, then the rule is, multiply the average illuminating power by 5, and divide by the actual rate; thus, supposing that the rate of burning was 5.1 cubic feet per hour in place of 5 cubic feet, then in the present example:

15.09
5

5.1)75-45

Corrected average 14.79

In order to correct for the consumption of the candles, multiply the average illuminating power (after correcting for the gas consumption if necessary) by the standard rate in seconds, and divide this result by the actual time of burning.

Thus, supposing in the present experiment that the candles occupied ten minutes fourteen seconds to burn the forty grains of sperm, in place of ten minutes (the standard time) then

[blocks in formation]

In order to correct for barometric pressure and temperature, multiply the corrected average as just obtained by 1,000, and divide the result by the tabular number in the Gas Referees' table constructed for this purpose, in line with the observed barometric pressure, and under the observed temperature, as indicated by the thermometer inserted in the meter.

There is another type of photometer also employed in gas testing, known as the "Evans," from the name of its inventor.

The type of photometer known as the "closed Evans"? was introduced in 1858. It consists of a long box enclosing the candles and the light to be tested. The disc was fixed and the candles were mounted on their balance, which moved in a travelling holder attached to an endless cord working with a small winch.

The only legal standard at present authorized for the testing of gas has long been considered unsatisfactory for many reasons, and at the present time a committee of specialists are studying the question as to which is the most reliable substitute.

The following remarks by Professor Lewes, who is the secretary of the committee who are investigating the question will be of interest to the student:

66

'Ever since Bouguer introduced the candle as a standard of light, it has held its own, in spite of the extremely unsatisfactory results which are to be obtained from it; and it is useless to strive after scientific accuracy with the remaining parts of the apparatus while the standard itself is liable to such wide variations as to absolutely nullify the results obtained by its use. Up to 1850, no definition seemed to exist as to what the candles were to be that were used for gas testing. In that year, however, an Act was passed in which wax candles were prescribed for this purpose; and in 1860 the Metropolis Gas Act defined and

prescribed sperm candles of six to the pound, each burning 120 grains per hour, in place of the wax candles previously used. These sperm candles emitted more light than the old wax candles-fourteen sperm candles being equal to sixteen wax ones. It was found, however, impossible to use pure spermaceti; and in order to prevent crystallization of the spermaceti, from 4 to 5 per cent. of beeswax had to be mixed with it. Slight variations in the quality of the sperm and the method of admixture, and changes in the process of manufacture, will sometimes tend to slightly alter the results obtained with such candles. The weak point of the candle, however, which has led to endless trouble, and which renders it absolutely unfitted to be looked upon as a standard of light, when used according to the ordinary prescribed methods, is the wick-the roughness or smoothness of which, and its curvature during burning, causing extraordinary variations in the light emitted by it, even supposing that the wicks could be made absolutely uniform, the twist of the thread kept constant, and the same amount of strain placed upon them during the making of the candle.

"It has been conclusively shown by Mr. W. C. Young that the standard sperm candle may be relied upon to give very uniform results, if due regard is paid to the condition and shape of the wick before commencing an experiment. But inasmuch as the method by which this desired end could be brought about is not in accordance with the instructions of the Gas Referees as to the mode of using the candle for testing gas, it is useless to discuss the question; and one can only agree with the various Committees who have considered the subject of standards of light, all of whom are unanimous in condemning the sperm candle as not being in any way reliable. In spite, however, of the torrent of abuse which during the past fourteen years has been showered upon the sperm candle and the numerous

« PreviousContinue »