When as has been thus prepared and purified, it has a composition variable as the coal used, and the beat and time of the operation. The composition marked No. 1 in Henry's table, is a very pure gas. During the present year, while public attention in New York was turned toward the cost and purity of gas, the two gas companies of that city (the Manhattan Co. and the New York Co.)jointly re quested Drs. Torrey, Ellet, and Chilton, to undertake a thorough investigation and analysis of the gases of both com panies, as well as that of the Philadel phia City Gas Co., and the report of these chemists appeared on the 22d May. The plan of their investigation and character of the experiments were those most likely to insure accuracy and involve means of detection not previously used in Europe. The following is an ab stract from their report:— " The following we believe represents the true constitution of the Philadelphia gas, as delivered to the consumers from April 15th to April 24th, and of the Man hattan and New York Companies' gases from April 24th to May 22d.
"All of the gases are effectually cleansed from carbonic acid and from sulphuret ted hydrogen, not the slightest traces of either of them being discernible by the most delicate re-agents. The lime puri fiers seem to perform their office tho roughly, and the gases have a less offen sive odor than was formerly the case. Ammonia has been detected in all of them, but the .quantity is very minute, and has not yet been estimated.
" It will be seen from the above results that the New York gaies resemble each other very closely, (as might have been anticipated,) since their manufacture is conducted as nearly as possible in the same way, and the materials employed in their production are the seine, viz., two-thirds of Cannel coal, and one-third of Newcastle coal. They have both an advantage over the Philadelphia gas in the greater relative quantity of their two most valuable constituents, viz., olcfiant gas, and hydro-carbon vapors.
"Photometrical processes are obviously the most reliable for determining. the relative illuminating value of different gases, and they accordingly have not been neglected by us. The comparisons have been made by means of a standard candle of Judd's manufacture, which itself has been compared with a standard candle used in the English gas works, and which had been employed within a few months in determining the relative value of their products. We have thus
been enabled to compare the illuminating values of the American gases on which we have experimented, not only with each other but also with those of Great Britain. It results from our experiments that no appreciable difference in this respect exists in the New York gases, the slight excess of olefiant gas and hy dro-carbon vapors in the one, being com pensated by the greater quantity of light carburetted hydrogen in the other.
"The quality of these gases we believe to be better than that of most of those manufactured in the principal cities of Great Britain, and if excelled by any of them, it is only by those which are de rived from pure Cannel coal.
" We find that the New York gases are of decidedly superior value to that manu factured in Philadelphia, and that to obtain a certain definite quantity of light, we must employ them in quantities represented by the following numbers—of Manhattan Gas, 100 cubic feet, New York Company's Gas 100 " Philadelphia Gas, ..... 152.3 " " " Me values of these gases as sources of light are of course inversely as these num bers.
" We do not attach much importance to specific gravity as exhibiting the worth of an illuminating gas. Carbonic oxide, and nitrogen, (the one of very little value, and the other absolutely worthless,) are always present in these gases, and are both identical in specific gravity with olefiant gas, which is one of. their most valuable constituents ; a circumstance which renders the test an unreliable one, except in connection with other charac ters. We have, however, made many trials on these gases, and have found that the specific gravity of the Philadelphia gas is below 450, while that of both the Manhattan and New York Companies' gases is on the average 550, atmospheric air being 1000.
Good gas furnished at moderate prices is still a desideratum, and has led to the use of camphene and water gas manufactured on a small scale. The cost of coal gas might be diminished by the sale of the waste substances produced as Coke, Sul phate of Ammonia, Gas Lime, and Tar. (See_0[1. GAS, RESIN GAS, AND WATER GAB.)