but the properties were the same, whichever of the three metals were used, and whether they were dissolved in sulphuric or muriatic acids. When the sulphuric acid was concentrated, iron and zinc dissolved in it with difficulty, and only by the assistance of heat. The air given out was not inflammable, but consisted of sulphurous acid in the state of gas. These facts induced Mr. Cavendish to conclude that the inflammable air evolved in the first case consisted of the unaltered phlogiston of the metals; that sulphurous acid consisted of the same phlogiston united to a portion of the acid, which deprived it of its inflammability. He tried the combustibility of inflammable air, when mixed with various proportions of common air, and found that it exploded with very great violence when mixed with about an equal bulk of common air.

He found that by the solution of copper in muriatic acid, with the assistance of heat, an air was obtained which lost its elasticity when it came in contact with water. This air, the nature of which Mr. Cavendish did not examine, was muriatic acid gas, the properties of which were investigated by Dr. Priestley.

The carbonic acid gas on which Mr. Cavendish made his experiments was obtained by dissolving marble in muriatic acid; and he found that it might be kept over mercury any length of time without undergoing any alteration, but that it was gradually absorbed by coming in contact with water. The whole air thus absorbed was separated again by exposing the water to a boiling heat, or by leaving it for some time in an open vessel. He discovered that fixed air is incapable of supporting combustion; and that common air, when mixed with it, supports combustion a much shorter time than when pure. The first experiments of this air are deserving of record. A wax taper burnt 80" in a receiver containing 180 ounce measures of common air: 'when the receiver was filled with a mixture of one part of fixed air and 19 of common air, the taper burnt 51′′. When the fixed air wasths of the whole mixture, the candle burnt 23"; and in proportion as the

quantity

quantity of fixed air was increased the candle burnt a shorter time, till at length it went out at the instant of immersion. Carbonate of potash was first obtained in the state of crystals by Dr. Black: Mr. Cavendish formed it by making a solution of pearl ashes absorb fixed air till it deposited crystals. He examined the properties of these crystals, and found that they were not altered by exposure to the air, did not deliquesce, and were soluble in about four times their weight of cold water.

Dr. Macbride had already ascertained, that vegetable and animal substances yield fixed air by putrefaction and fermentation. Mr. Cavendish found by experiment, that sugar, when dissolved in water and fermented, gives out about half its weight of fixed air, possessing exactly the properties of the fixed air from marble. During these, and many other experiments of the same sort, he investigated the properties of the gases obtained from various substances. He next turned his attention to mineral waters: what had already been done, was either inaccurate, or led to conclusions which chemists were unable to explain. One of these, and not the least puzzling, was the existence of a considerable quantity of calcareous earth in some mineral waters, which was precipitated by boiling the water. Nobody could understand how an insoluble substance, as carbonate of lime, could be held in solution, nor why it was thrown down on exposing the water to a boiling heat. To determine this point, Mr. Cavendish made experiments on water raised by a pump in Rathbone-Place, London, and found that, when boiled,it deposited a quantity of earthy matter consisting of lime and magnesia. These he soon found were held in solution by fixed air; and he demonstrated by experiment, that this gas has the property of holding lime and magnesia in solution when an excess of it is present.

Dr. Priestley discovered, that when nitrous gas is mixed with common air over water, a diminution of bulk takes place; that there is still a greater diminution of bulk when oxygen gas, or, as he called it, dephlogisticated air, is employed instead

of

of common air, and that the diminution is in proportion to the quantity of oxygen present in the gas mixed with the nitrous gas. This discovery induced him to employ nitrous gas as a test of the quantity of oxygen present in common air, And various instruments were contrived to facilitate the mixture of the gases, and the measurement of the condensation : these instruments were called eudiometers; of which one of the best was contrived by the Abbé Fontana. By the use of eudiometers it was soon inferred, that the proportion of oxygen or pure air varied in different places; and to this variation was ascribed the healthiness or unhealthiness of particular places. Mr. Cavendish examined this important subject, and ascertained, that the apparent variations were owing to inaccuracies in making the experiment; and that, when the requisite precautions were taken, the proportion of oxygen in air was found to be constant in all places, and at all seasons. He likewise found, that common atmospheric air is a mixture of nearly 21 parts by bulk of oxygen gas and 79 of azotic gas.

Philosophers had for many years maintained, that mercury was essentially liquid, and that it was incapable of congelation by any degree of cold whatever. It was accidentally discovered, that it may be frozen like other liquids: a fact that was at first generally discredited, but finally established by irrefragable experiments. According to the observations made by some chemists at Petersburgh, it was thought that the freezing point was not less than several hundred degrees below zero. It became an object of importance to determine the exact point at which this metal would become solid and malleable; which was first done at Hudson's Bay by Mr. Hutchins, who followed a set of directions given to him by Mr. Cavendish; and from these experiments Mr. Cavendish concluded, that the freezing point of mercury is about 59 or 40 degrees below the zero of Fahrenheit's scale.

Hence the attention of Mr. Cavendish was drawn to the phænomena of freezing, to the action of freezing mixtures, and the congelation of acids. He published, in the Philosophical

1

losophical Transactions, several very curious and important papers on these subjects, in which he explained the phænomena of congelation exactly according to the theory of Dr. Black; rejecting only the hypothesis, that heat is owing to the presence of a peculiar matter, and referring it, with sir Isaac Newton, to the rapid internal motion of the particles of the hot body. The latent heat of water he ascertained to be 150°. With respect to the nitric and sulphuric acids, he showed that their freezing points varied very considerably, according to the strength of each. The papers published in the Transactions of the Royal Society are thought to constitute one of the most interesting and best established parts of the theory of heat, as at present taught by chemical philosophers. But, says Dr. Thomson, in his Life of this illustrious chemist, the most splendid and valuable of Mr. Cavendish's chemical experiments were published in two papers, entitled Experiments on Air: the first inserted in the Philosophical Transactions, vol. lxxiv. and the second in the succeeding volume. The object of these experiments was to determine what happened during the phlogistication of air, as it was at that time termed; that is, the change which air underwent when metals were calcined in contact with it; when sulphur or phosphorus was burnt in it; and in other chemical processes. He showed that carbonic acid was not formed except when some animal or vegetable substance was present; that when hydrogen gas was burnt in contact with air or oxygen gas, it combined with that gas and formed water; that nitrous gas, by combining with the'oxygen of the atmosphere, formed nitrous acid; and that when oxygen and azotic gas are mixed in the requisite proportions, and electric sparks passed through the mixture, they combine and form nitric acid. The first of these opinions occasioned a controversy between Mr. Cavendish and Mr. Kirwan, who had maintained that carbonic acid is always produced when air is phlogisticated. "All the arguments of Kirwan," says Dr. Thomson, "are founded on the experiments of others: he displays great reading, and a strong memory; but does not discriminate between the merits of the chemists on whose authority he founds his own opinions. Mr. Cavendish, on the other hand, never advances a single opinion

opinion which he has not put to the test of experiment, and never suffers himself to go further than his experiments will warrant. Whatever is not accurately determined by unex ceptionable trials is merely stated as a conjecture, upon which little stress is laid." This controversy was carried on in the Philosophical Transactions, vol. lxxiv. and in the first of his papers Mr. Cavendish has drawn a comparison between the phlogistic and antiphlogistic theories; has shown that each is capable of explaining the phænomena of chemistry in a satisfactory manner; that it is impossible to demonstrate the truth of either; and he has given the reasons which induced him to prefer the phlogistic theory to the other, "which," says Dr. Thomson," the French chemists were unable to refute, and which they were wise enough not to notice. Nothing can be a more striking proof of the influence of fashion, even in science, and of the unwarrantable precipitation with which opinions are rejected or embraced by philosophers, than the total inattention paid by the chemical world to this admirable paper." "Such," continues our author, "were the chemical papers published by Mr. Cavendish. They contain five notable discoveries; all of them brought nearly to perfection by that illustrious author. These are, 1. The nature and properties of hydrogen gas. 2. The solvent of lime in water when the lime is deposited by boiling. 3. The exact proportion of the constituents of atmospherical air, and the fact that these constituents never sensibly vary. 4. The composition of water: and 5. The composition of nitric acid.

It was Mr. Cavendish's paper, printed in 1766, on fixed and inflammable air, that introduced a style of experimenting in pneumatic chemistry, more neat, more precise and scientific than had before been known. The attention of Dr. Priestley, to whose discoveries we devote the remainder of this Introduction, however, was not originally excited by the works of his predecessors, bu. by the accident of his living near a brewhouse at Leeds, where he had an opportunity of obtain ing fixed air on a large scale. One experiment led to another, till the fruits of his amusements were the discoveries on which his philosophical reputation was principally founded.

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