a manner perfectly free from all sources of error, would require such an apparatus, as few beside adepts in chemistry are likely to possess. The apparatus adapted to this purpose is described in the 6th chapter of Lavoisier's Elements. The fact may, however, be shown, less accurately, by the combustion of phosphorus, in the manner which has been already described. The first effect of the combustion will be a depression of the water within the jar; but when the combustion has closed, and the vessel has cooled, a considerable absorption will be found to have ensued.

Those persons, who are possessed of a mercurial apparatus, may repeat this experiment in a less exceptionable manner. On the surface of the quicksilver let a small hemispherical cup float, made of untinned sheet-iron; and, in order to keep it from the sides of the jar, it may rest on a wire-stand, shaped like the figure 43, plate iv. Let a jar, the height and diameter of which must be regulated by the size of the mercurial trough, be filled with oxygen gas over water, and be removed, by means of a piece of pasteboard, as before described, to the mercurial bath, inverting it dexterously over the tin cup. If the phosphorus had been previously set on fire, a large quantity of the gas, expanded by the heat, would have escaped, and would have prevented the accurate measurement of the absorption. After drying the surface of the mercury within the jar by blotting paper, a portion of the included gas must, therefore, be removed. This is done by an inverted syphon, one leg of which is to be introduced (in the same manner as is shown at fig. 41, g) within the jar, before placing it over the mercury; and the gas will be forced through the open extremity of the other, when the jar is pressed down into the quicksilver. When the proper quantity has been expelled, remove the syphon. The cup, containing the phosphorus, will thus rest on the surface of the quicksilver within the jar, and above the level of the mercury without. The phosphorus is to be inflamed by passing a crooked iron wire, made red hot, through the quicksilver. On the first impression of the heat arising from its combustion, the included gas will be considerably expanded; but when the phosphorus has ceased to burn, a considerable absorption

will be found to have taken place, the amount of which may be measured by ascertaining the height of the quicksilver within the jar, before and after the experiment. The quantity of phosphorus employed should be very small, and should not bear a greater proportion than that of 10 grains to each pint of gas; otherwise the combustion will go on so far, as to endanger the breaking of the jar, by the approach of the inflamed phosphorus.

In this process, a dense white vapour is produced, which concretes on the inner surface of the jar in solid flakes. This substance has strongly acid properties; and, being formed by the union of oxygen with phosphorus, is termed the phosphoric acid.

The diminution of the volume of oxygen gas, by the combustion of other bodies, may be ascertained in a similar manner. When the substance employed is not easily set on fire, it is proper to enclose, along, and in contact with it, a small bit of phosphorus, the combustion of which excites sufficient heat to inflame iron-turnings, charcoal, &c. In the instance of charcoal, however, though that substance undergoes combustion, no absorption ensues; because, as will appear in the sequel, the product is a gas, occupying exactly the same bulk as the oxygen gas submitted to experiment.

IV. During the combustion of bodies in oxygen gas, a large quantity of caloric is liberated.-Lavoisier has endeavoured to prove (Elements of Chemistry, chap. ix.) that a given weight of oxygen abandons very different quantities of heat, when combined with different inflammable bodies. For example, the caloric disengaged from 1 pound of oxygen, during the combustion of its equivalent of phosphorus, he estimates to be sufficient to melt between 66 and 67 pounds of ice; of charcoal, between 37 and 38 pounds; and of hydrogen, 52 pounds of ice. There can be little doubt that the heat, evolved in these combustions, as well as the light, has its origin partly from the oxygen gas, and partly from the combustible body; but the precise quantity due to each can scarcely be considered as yet determined. Nor must it be understood, that the transition of oxygen from the gaseous to a more dense state is essential to the phenomena of combustion; for besides that

we have several examples of true combustion where oxygen is not at all concerned, as in those effected by chlorine, the principle must now be still more limited by a variety of cases, to be afterwards mentioned, which show that active and vivid combustion is sometimes attended, not with a condensation of the bodies that combine, but on the contrary with a great enlargement of volume; and, what is still more remarkable, that in some instances the new compounds, formed by combustion, have as great a capacity for heat as their constituent principles. (See Petit and Dulong's Memoir. Ann. de Ch. et Phys. x. 395.)

V. All bodies, by combustion in oxygen gas, acquire an addition to their weight; and the increase is in proportion to the quantity of gas absorbed, viz. about one third of a grain for every cubic inch of gas.-To prove this by experiment, requires also a complicated apparatus. But sufficient evidence of the general fact of an increase of weight may be obtained by the following very simple experiment. Fill the bowl of a tobacco pipe with iron wire coiled spirally, and of known weight: let the end of the pipe be slipped into a brass tube, which is screwed to a bladder filled with oxygen gas: heat the bowl of the pipe, and its contents, to redness in the fire, and then force through it a stream of oxygen gas from the bladder. The iron wire will burn; will be rapidly oxydized; and will be found, when weighed, to be considerably heavier than before. When completely oxydized in this mode, 100 parts of iron wire gain an addition of about 30.

VI. The substances, capable of uniting with oxygen, afford one or other of the following products; 1st, an acid; 2dly, an alkali or earth; or 3dly, an oxide. It is not easy to give precise definitions of these three classes of compounds. Of the acids, for instance, there are some which, though entitled to that epithet by their general qualities of chemical combination, are not sour to the taste; and others which do not redden vegetable blue colours. Nor is oxygen an element essential to the acidity of a compound, for some bodies are rendered acid by union with chlorine, and others by combination with hydrogen. The theory of Lavoisier, therefore, which considered oxygen as the essential principle of acidity,

and in conformity to which its present name was assigned to it,* can no longer be received as correct. The name, however, may be retained, to avoid the inconvenience of changing one so long and generally received.

Alkalis and earths are chiefly distinguished by acting as bases, with which the acids combine, with the loss generally of the separate characters of each. The alkalis are soluble in water, and change some vegetable blues to green; and the earths are either not soluble at all or sparingly soluble in that fluid; but in the quality of insolubility they agree with many of the third class of compounds, viz. oxides, especially with those derived from the metals, which also serve as bases to the acids.

In many instances, a combustible body, which affords an acid when united with a certain quantity of oxygen, gives an oxide when combined with a less quantity; and the acid may be brought back to the state of an oxide by separating part of its oxygen. We have examples, also, furnished chiefly by a few of the metals, in which the same body, combined with a small proportion of oxygen, gives an oxide that is capable of uniting with acids, and of composing salts, and again, united with more oxygen, yields an acid which is susceptible, with alkalis and earths, of forming saline compounds.

VII. Oxygen gas supports, eminently, animal life.—It will be found that a mouse, a bird, or other small animal, will live four or five times longer in a vessel of oxygen gas, than another animal of the same kind and size will live in a jar of atmospherical air of the same dimensions.

VIII. This effect seems connected with the absorption of oxygen by the blood.-Pass up a little dark-coloured blood into a jar partly filled with oxygen gas, and standing over mercury. The gas will be in part absorbed, and the colour of the blood will be changed to a bright and florid red. This change to red may be shown, by putting a little blood into a common vial filled with oxygen gas, and shaking it in contact with the gas.

• From ovc, acid, and yeıvoμai, to generate.

SECTION II.

Of Chlorine.

Chlorine was discovered by Scheele in the year 1774, and first described by him in an Essay on Manganese, under the name of dephlogisticated marine acid. It was afterwards termed in the French nomenclature oxygenated muriatic acid, and by Dr. Pearson oxymuriatic acid, a name generally received in this country, till superseded by that which it now almost universally bears. The simplest state, in which we are able to examine its properties, is that of a gas.

I. This gas may be formed by either of the following pro

cesses:

Process 1. Into a stoppered retort introduce eight ounces of liquid muriatic acid, sp. gr. 1.160, and four ounces of finely powdered manganese, and apply the heat of a lamp. A gas will be produced, which may be received, in the usual manner, over water of the temperature of 80° or 90° Fahrenheit. From the foregoing materials about 160 cubical inches may be obtained.

Process 2. Grind together in a mortar eight ounces of muriate of soda (common salt) with three ounces of powdered manganese; put them into a stoppered retort, and pour on them four ounces of sulphuric acid, which have been diluted previously with four ounces of water, and suffered to cool after dilution. Or the proportions recommended by Thenard may be employed, viz. 4 parts of muriate of soda, 1 part of oxide of manganese, water and sulphuric acid each 2 parts. On applying a gentle heat, gas will be produced, as in Process 1. But as the gas is absorbed by contact with cold water, though not rapidly, it should be received, when it is intended to be kept, in bottles filled with, and inverted in, water of the temperature of 80° or 90° Fahr. and provided with accurately ground stoppers. It will be found also much to diminish the loss of gas by absorption, if it be made to issue from a gas bottle, the