Another valuable product is the tar which is deposited during the production of the gas, this tar when rectified by a slight evaporation, has become an article of commerce. Large establishments, both of coal tar, coal oil, and pitch, are in full action, and the commodities which they furnish have become in great demand. The ammoniacal liquor which the gas-light process affords, has of late given rise to very important branches of chemical manufacture, carried on upon a large scale. But as the gas is at present supposed to be the only object in view, for the sake of the light which it yields, the other products being only accidentally connected with its extraction, let us leave the idea of profit on them out of the question, and with the utmost latitude of concession, require them only to stand as in part for a portion of the coal employed in the process, we have still the gas, an article which performs the functions of the oil, the tallow, or the wax for which it is substituted; and to the price of which we have no need to call the attention of those who make use of them. There remains only to be opposed on the other side, the expence of the apparatus by which the gas is to be prepared, and the lights maintained. From the materials and the workmanship, with the interest of the capital sunk, the expence in the first instance, must be very considerable. But where the quantity of light must be great, even from cheap substances, or where, with a less quantity of light, the substances from which it is derived must be of the costliest kind; such is in either case the enormous expence of these materials, that by superseding them and making every reasonable allowance to the engineer who erects the gas apparatus, the sum it costs, both principal and interest, is soon liquidated, leaving at last a total saving, excepting the expence of accidental repairs, which, from the durability of the materials employed, seldom exceeds a trifling sum. The principal expence in the pursuit of this new branch of civil and domestic economy, is therefore, the dead capital employed in erecting the machinery for obtaining and conveying the gas. The floating capital, after the first cost incurred in erecting the apparatus, is comparatively small; even if usurious interest is allowed for the first cost of the apparatus, and its deterioration, the saving must always be considerable, especially if the number of lights furnished are comparatively in a small place.