Utility of conversation flags.

versation flags, and on signal being made, down came four boats to take us up to Canton, where otherwise we should not have reached until the next tide.

"Sometimes a boat cannot live upon the sea, and it may blow a gale, which prevents speaking. Our Conversation Flags, in such cases, obviate all difficulties."

COMBUSTION.

BY R. A. COFFIN.

Queries suggested.

Former views on the subject.

THE fact that fire will not burn without the presence of air, is universally known. Perhaps in many minds, while contemplating this fact, such inquiries as the following have arisen :-What is the reason that the presence of air is necessary?-How does the air act on the fire to make it burn?-Does the fire produce any effect on the air which comes in contact with it? and if so, what effect?-What is flame ?-how is it produced? and what becomes of it, when extinguished? In many instances, probably, the inquirer has found himself unable to obtain any satisfactory answer to his questions. But it is by inquiries like these that truth is discovered.

For a long period, the views which prevailed among philosophers, in regard to the nature of that operation which we call combustion, as well as to the laws by which it is regulated, were very erroneous. The name

of phlogiston was given to the agent by which the process of combustion was supposed to be effected. Hence this theory is often called the phlogistic theory. Every combustible body, said the advocates of this theory, contains a quantity of phlogiston. Some bodies are more combustible than others, because they contain more phlogiston, or give it out more readily. Some bodies contain no phlogiston, and are therefore wholly incombustible. When a

Phlogistic theory.

Discovery of oxygen.

body burns, it gives out phlogiston. The light and heat are occasioned by the violent agitation of the phlogiston, at the moment of its escape from the burning body. The reason why a body which has been burnt once cannot be burnt again is, that it has lost all its phlogiston.-Such was the theory which philosophers taught, and in which their hearers for a long time acquiesced. Without examination-without experiments-the existence of phlogiston was assumed, and on this assumption, a theory was framed, by which it was supposed that all the phenomena of combustion could be explained. No one could tell what were the properties of this powerful agent, for no one had ever obtained it, or even seen it. Still, however, its existence and influence were spoken of with as much confidence as if it were equally well known with air or

water.

The discovery of oxygen by Dr. Priestly, in 1774, prepared the way for the overthrow of the phlogistic theory; and the experiments of Lavoisier, made not long after, proved that burning bodies, instead of giving out phlogis ton, absorb oxygen. In order to a clear understanding of the philosophy of combustion, it is necessary to know the leading properties and characteristics of oxygen; and these, therefore, will now be stated.

Oxygen is a colorless, tasteless and inodorous gas. It is perfectly invisible when in a state of gas-and this is the only state in which it has ever been obtained pure. It is a little heavier than atmospheric air, about in the proportion of 1 to 1. It forms about one fifth of the atmosphere, and eight ninths of water; and enters more or less into the composition of nearly all vegetable and animal substances. Many minerals contain it in large quantities. In short, no other material substance is so extensively diffused as oxygen. The most striking prop

What is meant by combustion, &c.

Properties of oxygen. erty of this remarkable substance is its power of exciting and supporting combustion. A candle or wax taper, freshly extinguished, is relighted on being immersed in a bottle of this gas. A watch spring, or piece of iron wire, will burn as readily in oxygen as paper will in common air. All that is necessary to exhibit this phenomenon is, to affix a piece of tinder to the watch spring or the wire, light it, and immerse it in the gas. The metal will immediately take fire, and burn with a most beautiful and brilliant light. Oxygen is never found in nature in its pure, uncombined state. In many cases, however, it is not difficult to separate it from the substances with which it is combined, and thus to obtain it in sufficient quantity, and of requisite purity, for performing various interesting experiments. The substances from which it is most generally obtained are oxyde of manganese, chlorate of potash, red lead and nitre. By strongly heating these, the gas will be given off.

As the terms combustible and supporter of combustion will frequently be used in the following pages, it may be well briefly to state the meaning of these terms, and the theory which gave rise to the use of them. Substances which will burn are called combustible; those which will not burn, themselves, but in which other substances will burn, are called supporters of combustion. Thus hydrogen gas is called a combustible, but not a supporter of combustion; because it will burn itself, but other substances will not burn in it. Immerse a candle in this gas, and it will be extinguished, while, at the same time, it will set fire to the gas. It was formerly supposed that the properties of a combustible body, and those of a supporter of combustion, were so utterly incompatible with each other, that they could not exist together in the same body. This supposition is now known to be

Combustion defined.

Supporters of combustion.

incorrect, as there are some substances which will both burn themselves and allow others to burn in them. It is not, therefore, strictly proper to regard combustibles and supporters of combustion as two distinct classes of bodies. Still, however, this classification is often made, and as these terms are frequently found to be convenient, they may perhaps be used without injury, except in cases in which great accuracy is required. In accordance with these views, and with sufficient accuracy for our present purpose, combustion may be defined, the combination of a supporter of combustion with a combustible body, attended with the disengagement of light and heat.

For a long time after the discovery of oxygen, it was supposed to be the only supporter of combustion in existence. The researches of modern chemists have, however, brought to light several other substances which appear to possess the same property; though it is supposed by some that these substances contain oxygen, and that to this they owe their power of supporting combustion. However this may be, they are of little importance, in comparison with oxygen; and in a sketch like this, it is perhaps sufficient merely to give their names. These are chlorine, iodine, fluorine and bromine. One or two others may possibly deserve to be reckoned in the list, but their claim is at least questionable. In fact, all common cases of combustion are produced by the union of oxygen with a combustible substance. If a candle be covered with a tumbler, or anything else, in such a manner as to prevent the access of air, it will burn for a short time, and then go out; and the time that it continues to burn will be found to depend on the quantity of air enclosed with it. Some substances have so strong a tendency to combine with oxygen, that when made to burn in a confined portion of air, they will continue to