LECTURE XXV.

CHEMISTRY.

GENERAL PRINCIPLES.

CHEMISTRY has been termed with some propriety the anatomy of matter; and its object is to discover the component parts of bodies, and, if necessary, to form them into new combinations. This last, indeed, is the principal occupation of the practical or trading chemist.

It is by the agency of heat and mixture, or, in other words, by the action of the particles of one body upon those of another, that the chemist is enabled to analyse or decompose the different substances which nature presents to his view. The matter of heat, or calorie, is the most powerful agent; for it has an attraction for most bodies, enters into their pores, and by its repulsive powers produces a decomposition of their constituent particles. There exists also in many cases a reciprocal attraction between the particles of one substance and those of another: this is seen particularly in all cases of solution; as when a lump of loaf sugar is put into a glass of water, it is said to be dissolved; that is, the particles of the sugar are intimately blended and united with those of the water by a mutual attraction.

Here let it be observed that mere mechanical mixture is very different from solution. A stone or an earth may be reduced to powder and mixed with water for a certain time, but the mixture will be muddy, and in a little time the solid particles will sink to the bottom; whereas if salt or sugar be dissolved in water, the compound will be always clear, and cannot be separated otherwise than by a chemical process.

These processes are generally by means of heat. -They are chiefly evaporation, or sublimation, which will be explained in the next lecture.

Fluid matters in which others are dissolved are called menstruu; and there is a certain limited quantity of the solid which they will dissolve or take up in solution by the power of attraction. Thus the quantity of salt which water will actually dissolve is limited, and whatever is beyond that will sink undissolved to the bottom. This point or limit is called the point of saturation.

Chemical attraction differs from the attraction of cohesion or aggregation in this, that it exists between the particles of bodies differing from each other, as between the salt and water; whereas the attraction of aggregation operates only between particles of the same kind. It is found also, that in chemical attraction the particles of certain bodies will be attracted by a given menstruum in preference to those of others. Thus sulphuric acid will dissolve a certain portion of copper, but if irou is added it will let fall the

copper and take up the iron, and if an alkali is added it will drop the iron and unite with the alkali. The body thus let fall is said to be precipitated, and is called a precipitate, and the substance which is employed to cause its precipitation is called a reagent.

These observations may serve to afford a general idea of the effects of heat and mixture. In the remainder of this lecture I shall endeavour to give an outline of the principal doctrines of modern chemistry, and particularly with respect to the simple substances. I shall in the two following lectures proceed to point out more particularly the chemical effects of heat and mixture. I shall next describe the chemical apparatus or instruments; and shall then briefly examine and explain the nature and properties of mineral substances, salts, earths, metals, combustibles, and waters, as far as chemistry has made us acquainted with them; and lastly shall treat of vegetable and animai substances in the same manner, particularly as to the nature of their constituent parts.

It is one of the great uses of philosophy to make us better acquainted with an intelligent Providence, to show how wisely and with what infinite design all things in the heavens and on the earth are disposed; and every step we advance affords fresh cause for admiration. You have just been contemplating the wonders of the firmament. Those which modern chemistry displays to your view, though on a minuter scale, are not less strik