negative definition of the Matter which he supposes the elementary principle of body. On this account, though to the imagination his theory may want the charms which the other possesses, yet it has the advantage of going just to the extent to which our perceptions or our observations authorize us to proceed, and of being accurately circumscribed by the limits pointed out by the laws of philosophical induction. *

• Though Boscovich's Theory was published long before Dr Hutton's, so early, indeed, as the year 1758, there is no reason to think that the latter was in any degree suggested by the former. Boscovich's theory was hardly known in this country till about the year 1770, and the first sketches of Dr Hutton's theory are of a much older date. Besides, the method of reasoning pursued by the authors is quite different; and their conclusions, though alike in some things, directly contrary in others, as in what regards gravity, inertia, &c. The Monads of Leibnitz might more reasonably be supposed to have pointed out to Dr Hutton the necessity of supposing the elements of body to be unextended, if the originality of his own conceptions, and the little regard he paid to authority in matters of theory, did not relieve us from the necessity of looking to others for the sources of his opinions.

The principal defect of his theory seems to me to consist in this, that it does not state with precision the difference between the constitution of those powers which simply form matter, and those that form the more complex substance, body. In other words, it does not explain what must be added to matter to make it body. The answer seems to me to be, that the addition of a repelling power, in all directions,

The existence of matter neither heavy nor inert, which he had taken so much pains to establish, was applied by him to explain the phenomena of light, heat, and electricity. He considered all these three as modifications of the solar substance, and thought that many of the appearances they exhibit, are only to be explained on the supposition that they consist of an expansive force, of which inertness is not predicable; in particular, that light is a power propagated from the sun in all directions, like gravity, with this difference, that it is repulsive, while gravity is attractive, and requires time for its transmission, which the latter does not, at least in any sensible quantity. *

The prosecution of this subject has led him to consider the nature of Phlogiston, a substance once so famous in chemistry, but of which the name has almost as entirely disappeared from the vocabulary of that science, as the word Vortex from the lan

is sufficient for that purpose. Such a repulsion, if strong enough, would produce both impenetrability and inertia. The matter, again, that possessed only an attractive power, like gravity, or a repulsive power only in a certain direction, like light, would not be inert nor impenetrable. In this inference, however, from his system, I am not sure if I should meet with the author's approbation.

* See Dissertations V. and VI. on Matter and Motion, in the work above quoted. The chemical Dissertation on Phlogiston is in the same volume, p. 171.

guage of physical astronomy. The new and important experiments made on the calcination of metals, and on the composition of water, are, as is well known, the foundations of the antiphlogistic theory. Nobody was more pleased than Dr Hutton with these experiments, nor held in higher estimation the character and abilities of the chemists and philosophers by whom they were conducted. He was nevertheless of opinion, that the conclusions drawn from them are not altogether unexceptionable, nor deduced with a sufficient attention to every circumstance. This remark he thought peculiarly applicable to what regards the composition of water, to the phenomena of which experiment, the dissertation we are now speaking of is chiefly directed. The two aëriform fluids, it is there observed, which compose water, in order to unite, must not simply be brought together, for in that state they might remain for ever unchanged, but they must be set on fire, and made to burn, and from this burning there are evidently two substances which make their escape, namely, Light and Heat. Though, therefore, the weight of the water generated, and of the gases combined, may be admitted to be equal, yet it must be acknowledged that two substances are lost, which the chemist cannot confine in his closest vessel, nor weigh in his finest balance, and it is going much farther than we are authorized to do, either by experiment or

analogy, to conclude that these substances have had no effect. As heat and light, in Dr Hutton's system, are composed of that matter which does not gravitate, the exact coincidence which M. Lavoisier observed between the weight of the water produced and that of the two elastic fluids, united in the composition of it, was no argument, in his eyes, against the escape of a very essential part of the ingredients.

Pursuing the same reasoning, he shows how little ground there is to suppose that the heat and light evolved in this experiment proceed from the vital air; and he concludes, that the real explanation of the process is, that by burning, the matter of light and heat, or the phlogiston of the hydrogenous gas, is set at liberty, and is thus enabled to unite with the vital air.

In the same manner, on examining what relates to the burning of inflammable bodies, he finds the oxygenous gas unequal to the effect of furnishing by its latent heat, or caloric, the whole of the sensible heat that is produced. He concludes, therefore, that the hypothesis of the existence of phlogiston in those bodies that are termed inflammable, is necessary to account for the phenomena of burning; phenomena, as he justly remarks, which are among the most curious and important of any that are exhibited by the material world. On the whole, it cannot be doubted, that great ingenuity

and much sound argument are displayed throughout the whole of this dissertation, and that whatever be ultimately decided with regard to the principle for which the author so strenuously contends, he has made it evident, that the conclusions of the antiphlogistic theory have been drawn with too much precipitancy, and carried farther than is warranted by the strict rules of inductive philosophy.

The subject of Fire, Light, and Heat, was resumed by Dr Hutton several years after this period, and formed the subject of a series of papers which he read in the Royal Society of Edinburgh, and afterwards published separately. He there explains more fully his notion of the substances just mentioned, which he considers as different modifications of the solar matter, alike destitute of inertness and of gravity.

A more voluminous work from Dr Hutton's pen made its appearance soon after the Physical Dissertations, viz. An Investigation of the Principles of Knowledge, and of the Progress of Reason from Sense to Science and Philosophy, in three volumes quarto.

He informs us himself of the train of thought by which he was led to the metaphysical speculations contained in these volumes. He had satisfied himself, by his physical investigations, that body is not what it is conceived by us to be, a thing necessarily possessing volume, figure, and impenetrability, but