人 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 produced 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 menstruums; 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. The particles of certain bodies are chemically attracted by certain menstruums more strongly than by others. Thus sulphuric acid will dissolve a certain portion of copper, but if iron 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 animal 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 striking. Can it fail to surprise any person capable of reflection, when he is told that the endless variety of created beings which nature presents to his view are composed from not more than forty-two simple substances; and that into the composition of the greater number not more most frequently than six or seven of these elementary substances are known to enter; for the metals and most of the earths are peculiar substances, and have little share in the formation of animals and vegetables. Let it be observed, again, that chemical analysis has not yet probably proceeded all the lengths at which it may hereafter arrive; that many substances which at present we regard as simple, may hereafter be proved to be compound, and that possibly the ingenuity of man may never be able to reach the ultimate and elementary particles of bodies. Those matters, however, which chemists have not as yet been able to decompose, they have a right to treat as simple substances. They are as follows: 1 Caloric, or the matter of heat and fire*. * I am aware of chemists of high character accounting caloric and light as distinct substances, and of the curious experiment of Dr. Herschell on which this opinion is founded. But I have some objections to that experiment not proper to be stated here; and we know so little of the chemical properties of light, considering it even as a distinct substance, that its introduction at present would only confuse the student. 10 Earths, of which there are nine in number. 11 Metals, in number about twenty-three. I. CALORIC* or elementary fire. That this is a fluid of a peculiar kind can no longer be doubted, since it has all the properties of a fluid. It is perceptible to our senses only in a disengaged or active state; that is, in passing from one body to another. It is, however, diffused very copiously throughout nature. By its elastic quality it is the cause of all fluidity; and, indeed, was it not for the influence of this subtile fluid, the whole matter of the universe, there is reason to believe, would be condensed into a solid mass. Thus it is that by withdrawing a certain portion of its natural heat from water, that fluid becomes a solid body and is converted into ice. Caloric might perhaps be termed the principle of *I adopt the term of the French chemists in preference to those of, fire or heat, because these are properly effects, and because caloric is known to exist where neither of these effects is sensible. It seems indeed a kind of solecism to speak of latent heat in cold water. repulsion, since it counteracts that of attraction, and seems to keep the particles even of solid bodies from adhering too closely; for an increase in the quantity of caloric, we see, expands even the most solid substances, such as metals. Yet we do not consider it as in a state of combination even when present in solid bodies, and still less so when obvious to our senses as producing fire and heat. But it may fairly be considered as in a state of combination when its presence preserves a body in a fluid state, even in the temperature of our atmosphere, as is the case with water, quicksilver, &c. A certain portion (or, in the chemical language, dose) of caloric reduces a body to the state of an incompressible fluid; a further dose brings it to the state of an aëriform or gaseous fluid. Thus a certain portion of caloric reduces ice to the state of water, a larger dose makes it volatile in the state of vapour. Some bodies have a superior attraction for caloric in one state and some in another. Thus mercury will not part with its caloric and become solid till the temperature is reduced to 40° below 0.-Water will not part with it and become ice till the surrounding bodies are at 30°. But though water retains it obstinately in this state, and though it becomes vapour at 212°; yet it readily in this state parts with its caloric to any body colder than itself, and becomes water again. The gasses (or permanently elastic fluids) have |