Because of the larger quantity of carbon disengaged by the former; but if the mixture with oxygen be in such proportions as at once to burn the whole of the charcoal in its gaseous combinations, without previous deposition, the light becomes blue, and is greatly reduced in intensity.

Why is the flame of a candle of various colours? Because of the different stages of combustion in it. Thus, at the bottom part, where the inflammable gases are given off in the smallest quantity, and where they are most intimately mixed with the air, the combustion is at once complete, and the light is blue and faint; the centre part, where the particles of charcoal, owing to a less admixture of oxygen, are thrown off in a solid state, and become incandescent before they are finally burnt, is white and highly brilliant; and the upper, where the charcoal is in still greater quantity, and much of it finally escapes combustion, is red and dull, as is seen when a candle requires snuffing.

Dr. Ure, a few years since, made a set of experiments on the relative intensities of light, and duration of different candles, the result of which is contained in the following table:

Why do those flames, whose products are only gaseous matter, give very little light?

Because the quantity of light which flame emits is dependent upon the incandescence of minute particles of solid matter, which are thrown off during combus

tion. The light of a stream of ignited hydrogen is scarcely visible in the day-light; but if a small coil of platinum wire be suspended in it, or some solid body, in very fine powder, such as the oxide of zinc, be projected through it, it becomes very luminous.

OIL GAS.

Why is oil gas cheaper than coal gas?

Because purified coal gas seldom contains more than 40 per cent. in volume of olefiant gas, while oil gas generally affords about 75 per cent.: hence its superiority for burning, and the relatively small quantity consuined. Thus oil, by being allowed to trickle into a redhot retort, half filled with coke or pieces of brick, to increase the heated surface, is decomposed, and yields a large quantity of gas, which is much richer in carburetted hydrogen than coal gas, and therefore much better fitted for the purposes of illumination. It contains no mixture of sulphuretted hydrogen, and requires no other purification than passing through a refrigerator; and as less of it is required for any given quantity of light, the atmosphere of a room is less heated and contaminated by its combustion. It is, however, considerably more expensive than the gas from coal; although the first outlay of capital for a manufactory upon a large scale is less, on account of the smaller size of the necessary pipes and apparatus. The commonest whale oil, or even pilchard-dregs, quite unfit for burning in the usual way, afford abundance of excellent gas. A gallon of whale oil affords about 90 cubical feet of gas, of an average specific gravity of 0.900; and an argand burner, equal to seven candles, consumes a cubical foot and a half per hour. Its economy may be judged from the following table :

Mr. Brande adds, that a pint of the best sperm oil, weighing about 13 ounces, burns, in a well trimmed argand, about ten hours. And by a series of experiments, conducted with every requisite caution, he found, that to produce the light of ten wax candles for one hour, there were required—

2600 cubical inches of pure carburetted bydrogen, or olefiant gas.

4875. 13120.

oil gas. coal gas.

Why is oil used by the Portable Gas Company? Because they are enabled to compress the gaseous matter obtained from oil to about one-thirtieth of its volume, or into a certain new liquid compound, colourless or opalescent, yellow by transmitted, green by reflected light; and combustible, burning with a dense flame. Hence its portability. When the bottle containing it is opened, evaporation takes place from the surface of the liquid; but this vapour soon ceases, and the remainder is comparatively fixed.

Why is gas from wood but ill adapted for illumination?

Because it is so deficient in the compounds of carbon and hydrogen. In manufactories, however, of charcoal in iron retorts, for the making of gunpowder, the gas which is given off is led by a pipe under the cylinders, and is economically employed in maintaining their heat.

ROSIN GAS.

Why is rosin gas even more advantageous than oil gas? Because rosin is of lower price, and less liable to fluctuations of value, than oil: indeed, the cost of the gas is stated at one-fourth that of oil; and the illuminating power of rosin gas, when compared with that from coal, is as two and a half to one, while it is of greater purity than that from coal or oil.

For the origin of this improvement we are indebted to Mr. J. F. Daniell, the distinguished meteorologist: his mode of treating the rosin is, to dissolve by gentle

heat about 8 lbs. in a gallon of the essential oil, which is plentifully formed during the composition of oil for making gas, or of rosin itself. This solution was allowed to trickle into the heated retort half filled with coke: thus, from 1000 to 1200 cubit feet of gas are obtained from 1 cwt. of rosin, and rather more than the original quantity of volatile oil is condensed, which is again employed for the solution. Mr. Daniell patented this means about three years since, and an apparatus on the plan has been erected by M. Martineau for the London Institution.

The burners consume about 1000 cubit feet of gas per day, obtained by 100 lbs. of common rosin, at about 6s., dropped with oil of turpentine on heated iron cylinders, in the proportion of 10 gallons of turpentine to 100 lbs. rosin; but the cost of the turpentine is not included in the 6s., as the same oil may be used over and over again, for any length of time.

Mr. Brande thus illustrates the advantages of rosin gas:- "The sources of supply are as inexhaustible, and more generally distributed, than those of the coal; and the forests of America, France, Spain, and Italy, yield the turpentine in quantities only limited by the demand. Many large towns in this country, in America, France, Holland, and the Netherlands, have already adopted the use of this gas. The elegance and simplicity of the manufacture, and the comparatively small capital required for the erection of the works, will also give it the preference in the creation of new establishments."

Returning to the comparative value of different hydro-carburetted gases, for the purpose of illumination, it seems evident, from Dr. Henry's experience, that, whatever be their source or composition, it may be most accurately determined by the quantity of oxygen required to saturate equal volumes. In other words, quotes Mr. Brande, the illuminating powers of

the different gases will be proportioned to the number of volumes of the gaseous carbon condensed into one volume of the gas; and of these, the oxygen consumed, and the carbonic acid produced, afford an accurate

measure.

HYDROCYANIC ACID.

Why is hydrocyanic acid so called?

Because it consists of hydrogen, and a gaseous compound, cyanogen, so styled by M. Gay Lussac, because it is the principle which generates blue, from two Greek words, signifying the blue-maker: or it may be obtained by means of Prussian blue; whence it is also called prussic acid. The acid thus obtained, has a strong pungent odour, very like that of bitter almond; its taste is acrid, and it is highly poisonous, so that the inhalation of its vapour should be avoided. It volatilizes so rapidly as to freeze itself.

From the experiments of M. Majendie, it appears, that the pure hydrocyanic acid is the most violent of all poisons. When a rod dipped into it is brought in contact with the tongue of an animal, death ensues before the rod can be withdrawn. If a bird be held a moment over the mouth of a phial containing this acid, it dies. In the Annales de Chimie for 1814, we find this notice: M. B., Professor of Chemistry, left by accident upon a table, a flask containing alcohol impregnated with prussic acid; the servant, enticed by the agreeable flavour of the liquid, swallowed a small glass of it. In two minutes, she dropped down dead, as if struck with apoplexy.

Hydrocyanic acid may generally be detected by its very peculiar odour. Scheele supplies this test: to the suspected liquid add a solution of green vitriol or copperas, and afterwards drop in pure potassa in slight excess, and after a short exposure to the air, redissolve the precipitate in muriatic acid. If hydrocyanic acid be present, the tint of prussian blue will appear. By