Column 7 of Table II. gives the expenses of putting down apparatus and mains for candle power per hour, calculated on the following grounds: -

In Belgium the cost of establishing a public burner may be estimated at 125. (for private illumination, stations, rooms, etc, where more elegant appliances are used, the cost may rise to 20s. or 27s. 9c*. per burner), and main-5 tenance and interest may be taken at 10 per cent, of this amount, say 5l. 18s. 9d. per annum. This cost must be spread over the entire consumption, depending on the number of hours during which it is in use.

Assuming a minimum of 700 hours per annum, and a consumption of 250 litres per hour, it will be found that at 1 1/2d. per cub. metre, interest and maintenance must be valued at about 6 per cent, of the value of the gas.

The corresponding expenses for a regenerative Siemens or Wen ham lamp, which costs at least 47s. 6d. for a consumption of 250 litres per hour, may be calculated in the following manner, a saving of 50% of gas being supposed: -

I

II

The expense of maintenance and interest is found to be 17 %.

Table II. - Illuminating Gas

We have, therefore, calculated at this rate the expenses tabulated in column 7, Table II., for the illumination given by Siemens or Wenham lamps.

For Clara on d and Aver burners we have adopted 10 % of the gas consumed as representing the corresponding expenses. The rate of 6% remains for ordinary burners, the cost of which does not exceed 4s.

It must be borne in mind that the numbers should vary inversely with the number of hours during which the burners are employed, since the sources of expense remain almost constant whatever be the consumption of gas.

Before passing to the examination of some other modes of illumination, a word may be said on the subjects of recuperation and intense burners.

Table III. - Water Gas. Photometric Tests Of Some's Burner

Consumption per candle power per hour.... 20.20 litres,

Price of gas at Frankfort per cub. metre.... .712d.

Cost per candle power per hour...... • 0142d.

Working expenses, 10 per cent....... .00142d.

Total cost per candle power per hour..... .01567d.

Table IV. - Magnesium Lamps

Table V. - Electric Light - Arc Lamps. (Heim.)

Table VI. - Incandescent Lamps. (After Heim.)

Table VII. - Incandescent Lamps

(Chem. Trade Jl.)

The luminous intensity of a flame increases very rapidly with its temperature, and may be approximately represented by an expression of the form I = at, when I is the intensity of the flame, and t its temperature. The great increase of intensity obtained by superheating the air may thus be- conceived, when it becomes sufficient to sensibly increase the temperature of the flame and so produce a more efficacious combustion. In the Siemens and Wenham lamps the air may be heated to 400°-600°C.,but the apparatus soon wears out if the latter temperature be habitually employed.

To render the recuperation rational and efficacious it is essential that the apparatus be so arranged that the air is heated by the products of combustion, and not simply by the flame of the burner, for this can only give up part of its heat at the expense of its illuminating power.

It has not been found advantageous to strongly heat the gas itself; on the contrary, deposits are thus formed which rapidly block up the orifices by which it escapes from the burner.

For a given consumption of gas there is a definite supply of air which corresponds to a maximum of luminous intensity. The determination of this quantity is of special importance when ordinary burners with cold air are employed; but even when hot air is supplied it is still important to ascertain exactly the draught which corresponds to a maximum of illuminating power.

Everyone can convince himself of the effect of the draught or volume of air brought into contact with luminous flame on its illuminating power by simply placing a glass chimney on the top of the glass of an Argand burner, so that the height of the chimney is doubled. It will be observed that the flame immediately becomes lower and throws a less powerful light on surrounding objects, although it may itself become whiter. Inversely, the intensity of a flame, which is burning with an excess of air, may be increased by diminishing the supply of the latter. This increase continues until the flame becomes brown towards the top, after which further diminution of the air supply causes a rapid diminution in the brightness of the flame, the combustion then becoming incomplete.

This experiment justifies the conclusion that for burners with cold air the maximum of light corresponds to the minimum of air which permits of complete combustion, and that the yellow flame is more economical than a white flame obtained by means of an excess of cold air.

When the air arrives at the burner Seated to 500° C., the influence of the draught will, of course, not be so great, but it must be remembered that the temperature of the flame expressed in degrees is twice as great as this.

The economy effected by these intense flames, that is to say, flames produced by a large amount of gas, may be explained by the facts that, in the first place, the amount of air immediately surrounding the flame is smaller for an equal volume of flame than in the ordinary burner, and that the loss of heat is thus also rendered smaller; and that in the second place the surfaces and volumes of neighbouring parts of the apparatus so situated as to be capable of absorbing heat from the flame are also smaller in proportion to the gas consumed; finally,. a wide flame consists of an interior cone of gas, surrounded by a thicker incandescent layer (which must be traversed by the air) than is the case in a narrow flame burning the same volume of gas.

On account of these various circumstances the temperature of a large flame is appreciably higher than that of a small one, and hence arises its greater illuminating power. This luminous intensity therefore, results from a more effective combustion, which produces an increase in the light radiated from the flame.