All pipes adapted to the exterior of buildings, should be kept a little distance off from the wall, to prevent the wet lodging between the pipe and the surface to which they adapted.
Sheet iron mains for the interior of houses, are preferable to copper mains, provided the course of the main with regard to the position of the branch pipes, does not require too many angular directions, or circular bends.
PART XIV.
Illuminating power of Coal Gas, and quantity of Gas consumed in a given time, by different kinds of Burners, and Gas Lamps.
The illuminating power of coal gas, differs according to the nature of the coal from which it is obtained, and the manner in which it is purified, together with the quantity of naptha or essential oil chemically combined, or mechanically suspended in the gas. For if the gas be strongly agitated with water, its illuminating power is diminished. Coal gas, which abounds in olifiant gas or supercarburetted hydrogen possesses the greatest illuminating power, and hence carburetted hydrogen obtained from the decomposition of coal tar possesses a greater illuminating power than the gas obtained from the coals which produced the tar. The illuminating power of carburetted hydrogen obtained from coal tar when compared to the gas obtained from the best Newcastle coal is in the proportion as six to five. In fact the intensity of light evolved during the combustion of gazeous bodies composed of carbon, hydrogen, and oxigen, is always in the ratio of the quantity of carbon contained in equal quantities of the gazeous compound, and hence the gas from animal oil which is chiefly composed of supercarburetted hydrogen or olifiant gas, surpasses in illuminating power the gas obtained from coal.
Half a cubic foot of coal gas, obtained in the ordinary way of manufacturing coal gas, from Newcastle coal, is equal in illuminating power and duration of time, to the light produced by a tallow candle six in the pound, burning for one hour, and as such a tallow candle lasts five hours, therefore fifteen cubic feet of coal gas, are equal in value with regard to illuminating power to one pound of candles. And as 112 pounds of Newcastle coal produce by the new method of manufacturing coal gas, at least 550 cubic feet of gas, therefore the quantity of gas produced from a chaldron of Newcastle or Sunderland coal, (the minimum weight of which is 27 cwt.) is equal in illuminating power to 1000 pounds of tallow candles.
The illuminating power of coal gas may readily be ascertained. Though the eye is not fitted to judge of the proportional power of different lights, it can distinguish in many cases with sufficient precision where two similar surfaces are equally illuminated. As the lucid particles emitted from luminous bodies are darted in right lines, they must spread uniformly, and hence their density diminishes in the duplicate ratio of their distance. From the respective situations, therefore, of the centres of divergency, when the contrasted and illuminated surfaces become equally bright, we are enabled to compute their relative degrees of intensity. And for this purpose it is assumed as a principle, that the same quantity of light, diverging in all directions from a luminous body, remains undiminished in all distances from the centre of divergency.
Thus we must suppose, that the quantity of light falling on every object, is the same as would have fallen on the places occupied by the shadow; and if there were any doubt of the truth of the supposition, it might be confirmed by some simple experiment.