heat and light which it before contained in the gaseous state, were said to be evolved in the form of flame. But in this case several requisites are not fulfilled; the light depends upon the combustible, and not upon the quantity of oxygen consumed; and there are very numerous instances of combustion, in which, oxygen, instead of being solidified, becomes gaseous during the operation and, lastly, in others no oxygen is present. Combustion, therefore, cannot be regarded as dependent upon any peculiar principle or form of matter, but must be considered as a general result of intense chemical action. We quote this perspicuous theory of combustion from Mr. Brande; its dependence upon electricity has been already noticed, as well as several properties of oxygen, in connexion with the general subject of combustion. (See pp. 60 to 65.)

CHLORINE.

Why is chlorine so called?

Because its colour is green (from the Greek word for green).

Dr. Ure observes, "the introduction of this term, (chlorine) marks an era in chemical science. It originated from the masterly researches of Sir Humphry Davy, on the oxymuriatic gas of the French school; a substance, which, after resisting the most powerful means of decomposition which his sagacity could invent, or his ingenuity could apply, he declared to be, according to the true logic of chemistry, an elementary body, and not a compound of muriatic acid and oxygen, as was previously imagined, and as its name seemed to denote. He accordingly assigned to it the term chlorine, descriptive of its colour, a name now generally applied.”

Why are chlorine and its aqueous solution used to prevent infection?

Because chlorine has the power of decomposing the

nocuous compounds which produce contagion, and resolving them into others, which are harmless.

Even

when combined with lime, in the substance usually known as bleaching powder, chlorine retains this antiseptic power. For the purposes of fumigation, chlorine, liberated from manganese and muriatic acid, or manganese salt and sulphuric acid, may be diffused through the atmosphere of the infected chambers, or the infected goods may be exposed to it. In the same way, the offensive odour of dead bodies may be mitigated by sprinkling them with solution of chlorine. Chloride of lime has also been successfully used in cases of burns; to destroy the stench of bilge-water in ships, and to correct the confined air in their holds; as well as to destroy the fire-damp in mines.

Why is bleaching powder always used in a liquid form? Because it has the property of bleaching only when water is present. Thus, if a piece of dry litmus paper be introduced into a jar of dry chlorine, it will suffer no change; but if previously wetted, the colour will speedily disappear. The colours of printed calico may readily be discharged by the same means.

The addition of the water to the chloride of lime, (or bleaching powder) effects its partial composition :— one half of the chlorine leaves the lime, and dissolves in the water; and this is the bleaching liquid of the shops, which is sold at a high rate, although it cannot cost more than a farthing a gallon. Sometimes this fluid is applied immediately to the substance to be bleached, but sometimes a weak acid is added, to destroy the slight affinity of the chlorine for the lime, and you will see by this addition, how much the bleaching power of the fluid is increased. The manufactory of the chloride of lime is carried on on a large scale in the north of England, by passing chlorine into leaden chambers, containing hydrate of lime in fine powder.-Brande.

Scheele first remarked this bleaching property; Berthollet applied it to the art of bleaching in France; and from him Mr. Watt introduced its use into Great Britain.

BROMINE AND IODINE.

Why is Bromine so called?

Because of its origin from a Greek word signifying a strong disagreeable odour." In like manner, iodine is named from a Greek word signifying “violet-coloured," which distinguishes its vapour.

Bromine and iodine have only very lately been discovered, and they belong to that class of substances, with whose use in the economy of nature we are at present totally unacquainted, and which have not yet been applied to any practical purpose: nevertheless, they are objects of the greatest interest to the chemist; and the study of their properties is particularly instructive, on account of the analogy which subsists between them and chlorine.

Bromine probably exists in sea-water, but its relative proportion must be exceedingly minute. One hundred pounds of sea-water, taken up at Trieste, afforded 5 grains of bromide of sodium = 3.3 grains of bromine. Here the bromine is unaccompanied by any iodine; and the same appears to be the case with the waters of the Dead Sea. În the water of the Mediterranean, on the contrary, iodine is always found with bromine. It is most readily recognized by evaporating the water, so as to separate all its more ordinary crystallizable contents, reducing the remainder to a very small bulk, and dropping in a concentrated solution of chlorine. The appearance of a deep yellow tint announces bromine. Dr. Daubeny, Professor of chemistry at Oxford, has discovered iodine and bromine in several salt springs and mineral waters of this country; and the Professor conceives that his analyses will tend to throw some light on the connexion between the chemical con

stitution of mineral waters and their medicinal qualities. Bromine has also been obtained from the ashes of sea-weeds, and we shall presently have occasion to notice its connexion with the cause of the ocean's tint. Thus, it has been discovered, not only in the waters of the ocean, but in certain salt springs, in the ashes of marine plants, and, according to Mr. Brande, in those of some marine animals.

Bromine acts with energy on the animal functions. A drop, let fall into the beak of a bird, was sufficient to kill it. Iodine was accidentally discovered in 1812, by M. de Courtois, a manufacturer of salt-petre at Paris. In his processes for procuring soda from the ashes of sea-weeds, he found the metallic vessels much corroded, and in searching for the cause of the corrosion, he made this important discovery. But for this circumstance, merely accidental, one of the most curious of substances might have remained for ages unknown; since nature has not distributed it, either in a simple or compound state, through her different kingdoms, but has stored it up in what the Roman satirist considers as the most worthless of things-the vile sea-weed.-Ure.

Iodine has been successfully applied in cases of cancer and bronchocele.

HYDROGEN.

Why is hydrogen employed for filling air-balloons? Because it is much lighter than the atmosphere; the principle upon which balloons are constructed being, that a solid rises in the atmosphere with any given force, when its weight is less than the weight of the air which it displaces by the amount of that force. Hydrogen is the lightest substance in nature at present known, being 14.4 less dense than common air, 16 times less dense than oxygen, and 14 times less dense than nitrogen. Pure hydrogen gas is not, however, necessary to fill balloons; and carburetted hydrogen gas,

such as is used to light the streets, has been advantageously substituted.

Dr. Ure considers every cubic foot of gas included in a balloon, to have by itself a buoyancy of fully one ounce avoirdupoise in the atmosphere. Hence, a balloon of ten feet diameter will have an ascensional force of fully 524 oz. or 33 lbs. minus the weight of the 314 superficial feet of the varnished silk enveloping the gas; and one of 30 feet diameter, a buoyancy of fully 14,137 oz. or nearly 890 lbs. minus the weight of the 2,827 feet of envelope. The subject of balloons, generally, belongs to Pneumatics.

Hydrogen also forms a component of all vegetable and animal products, and is therefore abundantly diffused throughout nature. It may be respired for a short time, though it is instantly fatal to all small animals. M. Maunoir, after having breathed a quantity of pure hydrogen, found that his voice had become remarkably shrill. It is inflammable, and extinguishes flame. When pure, it burns quietly, with a lambent flame at the surface, in contact with air; but if mixed with thrice its volume of air, it burns rapidly, and with detonation.

THE BLOWPIPE.

Why is the gas blowpipe so important an instrument in chemical analysis, &c.?

Because it enables us to employ a mixture of oxygen and hydrogen gases, and thus to produce the most powerful heat yet known. This may be shown by preparing a bladder full of each of these airs, and forcing some out of each, into a common tube connected with both, and throwing a stream of the mixed gases on burning charcoal, or on any other substance in the act of combustion. These bladders should each be furnished with a small metallic pipe and stop-cock, and the tube connecting with both should have a very small orifice, in order that a regular stream of the commixed