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

this means one ten-thousandth part of the acid may be detected in water. Another test, in which copper is used, will, however, detect one twenty-thousandth of the acid in water. We must render the liquid containing the hydrocyanic acid slightly alkaline with potash, add a few drops of sulphate of copper, and afterwards sufficient muriatic acid to redissolve the excess of oxide of copper. The liquid will appear more or less milky, according to the quantity of hydrocyanic acid present. M. Orfila recommends nitrate of silver as a test, by which the acid will be precipitated in the form of cyanure of silver.

PROPERTIES OF METALS.

Why did the ancients designate the seven metals known to them by the names of the planets?

Because they were supposed to have some hidden relation: each being denoted by a particular symbol, representing both the planet and the metal.

Why are some metals called native?

Because they occur pure or alloyed, and have but a feeble attraction for oxygen; such as platinum, gold, silver, mercury, and copper. Metals are also found combined with simple supporters of combustion; the chief of these are metallic oxides. Metals combined with simple inflammables, include native metallic sulphurets. Metals in combination with acids, include metallic salts.-Abridged from Brande.

Why are some metals called native alloys?

Because they are found combined with other metals, Why are metals refined by fire?

Because advantage is taken of some property in

which the metal operated upon may differ from those with which it is alloyed, or from which it is desired to separate it. These differences may consist of the facility or difficulty of oxidation; in their tendency to volatilize; in the temperature required for fusion, and in their relative specific gravities.

Metals, when exposed to the action of oxygen, chlorine, or iodine, at an elevated temperature, generally take fire, and, combining with one or other of these three elementary dissolvents, in definite proportions, are converted into earthy or saline-looking bodies, devoid of metallic lustre and ductility, called oxides, chlorides, or iodides.

Why have the metals, as a class, a peculiar lustre? Because of their great power of reflecting light, in consequence of their opacity. Mr. Brande observes: "their opacity is such, that, when extended into the thinnest possible leaves, they transmit no light; silverleaf, only one hundred-thousandth of an inch in thickness, is perfectly opaque. Gold is perhaps the only exception, which, when beaten out into leaves one two-hundred-thousandth of an inch in thickness, transmits green rays of light."

Why are the polished metals peculiarly fit for burning mirrors?

Because they are very imperfect radiators and receivers of heat, but excellent reflectors both of light and heat.

POTASSIUM.

Why is potassium not found in an uncombined state? Because of its great affinity for oxygen. All that has hitherto been obtained has been procured by chemical means from the potash of commerce.

Why did Sir H. Davy discover potassium by the agency of Voltaic electricity upon pure potash?

Because he was thus enabled to detach the oxygen, and then the alkaline base appeared in small bubbles

having the lustre and outward characters of quicksilver.

Why is potassium importunt to the philosophical chemist?

Because it is capable of detecting and separating oxygen wherever it may exist, and however intimate and energetic may be the nature of its combinations. By its means water may be detached from the most highly rectified alcohol and ether; and, by its decomposition, hydrogen gas will be evolved. Potassium also combines with phosphorus, sulphur, and hydrogen; it forms metallic alloys with gold, silver, mercury, and some other metals; at a red heat it will decompose glass; and is even capable of reducing all the metallic oxides.-Parkes.

Potassium was discovered in 1807, by Sir Humphry Davy: he obtained it by submitting caustic potassa, or potash, to the action of Voltaic electricity; the metal was slowly evolved at the negative pole. Gay Lussac, and Thénard, first procured it by heating iron turnings to whiteness in a curved gun-barrel, and melting potash to come in contact with the turnings, when, air being excluded, potassium was formed, and collected in the cool part of the tube.

Why does potassium burn with a beautiful flame on

water?

Because the potassium decomposes the water and absorbs the oxygen, while the hydrogen of the water escapes and takes fire by the heat which the rapidity of the action produces.

Why does sulphuric acid, when dropped on chlorate of potassa, cause it to ignite?

Because of the evolution of oxide of chlorine. A mixture of sulphuret of antimony, and the salt suddenly deflagrates with a bright puff of flame and smoke. Matches tipped with this inflammable mixture are now in common use, and are inflamed by