neous: the three last have actually been decomposed into simpler substances.-The following are some of the most remarkable circumstances connected with chemical attraction. 1. All substances have not this affinity (or at least do not exert it) towards each other. Neither sand nor oil, for example, will combine with water; for, if they be blended however intimately by agitation, the one will fall to the bottom, and the other will rise to the top of the water.-2. It is almost essential to the operation of chemical attraction, that one at least of the substances be in a fluid state, otherwise the attraction of cohesion would prevail.-3. Chemical attraction is in many cases much affected by temperature. The addition of heat frequently enables substances to combine, which would not enter into combination at a lower temperature. This also is to be attributed to a diminution of the power of cohesion. -4. The proportions in which bodies will combine are very different in different substances. Some substances combine in all proportions; thus, if we take of water and spirit of wine any proportion, the two bodies will enter into combination. There is a second class of combinations, in which a fixed quantity of one of the substances will admit the other into combination in any quantity less than a certain extent, but not beyond it; thus a certain quantity of water will dissolve a certain quantity of salt or any less quantity, but will dissolve no more; when the water has dissolved all the salt which it can take in, it is then said to be saturated. There is a third class of combinations, in which the substances combine only in one certain proportion, and if there be more than the due proportion of either, the excess remains uncombined and unchanged: this is the case with some gases. There is still a fourth class, in which the substances combine in different proportions, but these are all definite, and, in the intermediate proportions no combination takes place; thus, there is a metallic substance called manganese, of which 100 parts will combine with 14 of a gas called oxygen, or with 28, or 42, or 56 parts, but will not combine with 30 or any other intermediate number of parts. It is a remarkable cir

cumstance connected with this class, that, in all cases, the second quantity of the smaller ingredient, that will combine with 100 parts, for example, of the greater, is precisely the double of the first, the third is triple, and so forth in arithmetical progression. Thus if 10 parts be the smallest quantity of one ingredient, that will combine with 100 parts of another, the next quantity of the former, which will so combine, is 20, the third is 30, and so on.-5. When three or more substances are brought within the sphere of chemical action, different results will ensue according to the different natures of the substances. In some cases, the whole ingredients enter into combination, and form one compound. In others, two of them combine, to the exclusion of the third. In a third class of cases, one is combined with each of the others. When a body composed of two substances, which may be called A and B, is added to another body composed of two different substances, which we call C and D, it very often happens that A quits B and combines with C, and B on the other hand combines with D; by which two new compounds are formed, AC and BD. Thus when a compound of sulphuric acid and soda is added to a compound of muriatic acid and lime, the soda combines with the muriatic acid, and the lime combines with the sulphuric.-6. Chemical combination is in general attended with a remarkable change in the properties of the bodies combined, so that we can seldom infer, from a knowledge of the combining parts, what will be the nature and properties of the compound. This is very remarkable in the combination of sulphuric acid and magnesia. The sulphuric acid tastes extremely sour, even when very much diluted with water, turns blue vegetable colours into red, and corrodes both animal and vegetable substances. The magnesia is a white powder, has no taste, will not dissolve in water, and turns blue vegetable colours into green. The compound, formed by the combination of these two very different ingredients in a certain proportion, has a bitter taste, has no effect on vegetable colours, will dissolve in water, and does not corrode either animal or vegetable substances.-Compounds of very different natures and

qualities will ensue from combinations of the same ingredients in different proportions. Thus the air we breathe and aquafortis, though substances widely dif ferent from each other, the one being the support of life, and the other a deadly poison,-are composed of the same ingredients in different proportions. When combination takes place in different determinate proportions, there is generally one of these in which the combining parts will neutralize each other, as it is called, that is, will destroy each other's characteristic properties; and in this case the two substances are said to be saturated with each other. Thus if an acid, which turns a vegetable blue into red, and an alkali, which turns a vegetable blue into green, be combined in one proportion, the compound will not at all affect the vege table colour; while, in another proportion, it will turn it into red, and, in another, into green. Bodies when combined, often lose the attraction which they formerly had for particular substances, and acquire for others an attraction which neither possessed before. Neither iron nor the gas called oxygen have any attraction for sulphuric acid while uncombined; but, when the iron and the gas are combined, the compound has a strong attraction for the acid. A change of form is a very frequent result of chemical action. This change is often very remarkable. Thus, by the combination of two gases, sometimes a liquid body is formed, and sometimes even a solid. Water is formed by the combination of two gases called oxygen and hydrogen, in the proportion of two measures of bulk of the latter to one of the former. So also a solid body is formed by the combination of two gases called muriatic acid and ammonia in equal measures.

COMPONENT PARTS OF ATMOSPHERIC AIR.

In the preceding article, it has been mentioned, that atmospheric air, which was so long regarded as an element, and still retains the name in ordinary language, has, in the present age, been clearly shown to be a compound substance. This discovery we owe to a philo

sopher of the name of Scheele. It consists of two elastic fluids, called the OXYGEN and NITROGEN gases, with which are mixed a small portion of another gas called carbonic acid, and vapour derived from the evaporation of water from the earth's surface. The carbonic acid gas, however, and the vapour are considered as having only an accidental connexion with the atmosphere, and not as essential constituent parts of it. The air may therefore be said to be composed of the oxygen and nitrogen gases, of which rather more than four-fifths are nitrogen. These substances are very dif ferent from each other in their qualities.-I. OXYGEN gas, though the smaller in point of quantity, is by far the more efficacious of the two ingredients of the atmo sphere. It is one of the most generally diffused and most powerful chemical agents in nature. It forms an essential component part of both air and of water, and is to be found in almost all animal, vegetable, and mineral substances. It is invisible, and has no taste nor smell. It is heavier than atmospheric air, being in the proportion of about 23 to 20. It is this ingredient, which gives the atmosphere its two most beneficial powers of supporting animal life and combustion. If the air be deprived of its oxygen, it is rendered quite unfit to maintain either respiration or combustion for a single moment. By each of these processes oxygen is consumed, and hence a frequent renewal of the air is absolutely necessary for the continuance of either. When a number of persons meet in a small room, they soon feel the necessity of admitting fresh air, and have recourse to open windows for this purpose. Upwards of a hundred of our countrymen on one occasion lost their lives, by being confined together for a single night at Calcutta, in consequence of the barbarous order of the Nabob, in a small apartment known by the name of the black-hole. You may perhaps be surprised to hear, that fish stand no less in need of oxygen than other animals, in order to support life. If several of them be confined in a small vessel, from which all communication with the external air is excluded, they first become much agitated, and at length expire. If a glass vessel be put over a candle, the light will become gradually feebler,

as the oxygen is consumed, and will at length die away. It is in consequence of the rapid supply of oxygen, that a fire burns so much more briskly, when exposed to a current of air.-II. NITROGEN gas, also known by the name of azotic gas, which is by far the larger ingredient of the atmosphere, is invisible, and has no taste nor smell. It is lighter than atmospheric air, being in the proportion of about 97 to 100. It neither supports respiration nor combustion; so that an animal immersed in it immediately expires, and a candle ceases to burn. Were it not for its combination with the oxygen, the air would be too pure, and, affording too free a respiration, would be more than the lungs are fit to bear.

The ATMOSPHERIC air, which is the result of the combination of these two gases, possesses the properties of the oxygen gas diluted with the nitrogen. It is invisible, and has no taste nor smell. Its specific gravity is little more than 1, if that of water be accounted 1000. It supports both respiration and combustion. The ingredients, of which the atmosphere is composed, when combined in different proportions, compose substances possessing very different properties. Thus we have seen in a former article, that, in one proportion,viz. 2 volumes of oxygen tol of nitrogen,-these ingredients compose nitric acid, well known by the name of aquafortis, a substance of a very corrosive and most deadly poisonous nature. In equal volumes these ingredients form nitric oxide, which is fatal to animal life, and extinguishes flame. Two volumes of nitrogen and one of oxygen form nitrous oxide gas, a substance remarkable for its intoxicating qualities, and hence called intoxicating gas, laughing gas, or gas of paradise. This gas, when inhaled, operates differently upon persons of different constitutions. The sensations produced by it are in general described as exquisitely pleasing,-an irresistible propensity to laughter,-a rapid flow of vivid ideas, a strong excitement to muscular motion,-joined to a singular thrilling in the ears, fingers, and toes. Mr Wedgewood, after throwing away the bag from him, continued for some time to perform the same motions, as if he had retained it, breathing on actively with an open mouth, and holding his nose with his fingers (as