and silversmiths in time, are obliged to use the utmost vigilance and care; but from the favourableness of our circumstances, we can manufacture coal gas at less expense, and consequently lessen the importance of a little extra consumption. Your surprise at the supposed alteration of my opinion is groundless, for, in the main, my opinion of oil-gas is the same. Indeed, some of its advantages are too obvious to be denied ; and I firmly believe, if the public can be made to pay for all the Oil-gas they consume, and in proportion to the quantity of light they received, there is little or no difference in point of economy to proprietors. "Wishing you all the success your talents, exertion, and perseverance demand,

"I remain, dear Sirs, yours truly,

"R. F."

"You say you have been complimented by Messrs. T. and M., for your conduct as to the formation of oil-gas. Taking into consideration all the circumstances relating to both gases, and from your experience, were you to seek a profit to yourself, and a benefit to the public, and you had a town. to light to-morrow, to which gas would you resort"-" To coal gas, certainly !”

(I.)

The following experiments to ascertain the average number of cubic feet of gas that may be obtained from a gallon of oil, and the relative illuminating power of coal and oil-gas, are alluded to by Mr. Lowe, in his evidence on the London and Westminster Oil-Gas Bill:

The total weight of oil used in the above four experi

ments, 111b. loz.; the total of gas obtained, 113 cubic feet; and the average specific gravity, 750.

5 13 9 31 | 955 | 35

The ratio of 64 cubic feet per gallon of oil.

From volatile oil only,

6 11 10 | 16

500

65

From a mixture of whale oil and volatile oil,

The ratio, 76 cubic feet per gallon of oil.

The average upon the whole of the above as five to three.

Extracts from the Evidence of Professor Leslie on the London and Oil-Gas Bill.

He considered the constitution of oil gas and coal gas as the same radically, only that the coal gas contained the larger, and the oil gas the smaller, proportion of hydrogen, in proportion to the mass. The proportions of carbon smaller in the coal gas and larger in the oil gas. The best coal gas may be made equal to the worst oil gas by increasing the richness and density, and consequently the illuminating power, of the coal gas. Oil gas is more subject so soot, if not completely consumed; and this is likely to happen both in public and private lighting, from inattention to the proportion of the orifice through which it flows, or the change of the gasometer.

That part of the carbonacious matter being more abundant in oil gas than in coal gas, and, therefore, not consumed with the same facility, a slight alteration in the mode of burning the gas, or a slight alteration in the pressure by which it is forced out of the gasometer, would produce a greater effect in producing smoke.

The specific gravity of the first oil gas produced to him was 674, but afterwards had some 985, and had fixed upon 900 as a favourable mean of oil

gas.

The specific gravity of coal varied much less than oil gas; he had some as low as 590, some as high as 620, in general about 600, which was the number he pitched upon as the specific gravity of the coal gas of Edinburgh, where the oil gas was 900, and therefore they were in the proportion of 2 to 3.

The specific gravity of the coal gas of London, was 410, and very steady; but the fluctuations in oil gas are very considerable, much more so than in coal gas.

The coal gas of Edinburgh is made of different materials from that at London; it is made from splint or parrot coal, a species of cannel coal, which yields no saleable coke.

He had made a considerable number of experiments upon the illuminating powers of oil and coal gas; the instrument he used for the purpose was a photometer, which is calculated to measure the intensity of light by the heat which is communicated to an opaque surface; and the results were, that, comparing the coal gas with the oil gas of Edinburgh, the former of the specific gravity of 600, the latter 900, they were as 2 to 3; but both were inferior in London, and not quite as 2 to 1; oil gas being as 20 to 11 of coal gas. He believed that oil gas was never made of

double the illuminating power of coal gas. He was aware that Dr. Brewster and others object to the use of the photometer to ascertain the degrees of light; that the ground of their objection was, that it would not accurately tell the various illuminating powers; and Dr. Brewster had asserted, that a heated iron ball, although not heated to ignition, will produce an effect upon the photometer; but it was a gross error, and Dr. Brewster had never tried the experiment, or he would have formed a very different result.

He found the heating power of coal gas about 10 per cent. greater at Edinburgh, and 20 per cent. in London, than oil gas-about 1th, viz., as 50 to 41.

With regard to the heat, he stated, that the supposition was, that the coal gas contained more, and oil gas less, hydrogen. In making that statement, he alluded merely to the mass or weight; that if equal bulks were taken, then it would be reversed; and the heating quality of the gas appeared to form an intermediate ratio between the weight and the bulk. In London the heating power of coal gas to oil gas, taking it in the one way, was five to four, and the other, eleven to sixteen: viz,, if you take equal bulks, the heating power of coal gas to oil gas was as five to four ; but if equal weights were taken, it was as eleven to sixteen. Oil gas being sold by meter, the same price is paid for gas of inferior quality as for good. If coal gas be charged £15 per 1000 cubic feet, oil gas ought to be £29, to be on an equal footing with coal gas.

He believed that both oil gas and coal gas may be made perfectly pure with due care, for the whole consisted in the subdivision of the gases, and their slow passage through the cream of lime; that if purified with due care, both were