On this last paragraph, however, it is obvious to remark, that the whole force of the argument rests upon an _insinuation_, that the cylinder and borer were _insulated_, or cut off from all communication with any other material substance. Had this been the case, then no doubt it would follow that an _endless_ supply of any thing _material_ could not be furnished by them; but if, as Dr. Boerhaave and many other learned and intelligent persons have supposed, fire be an element universally present, and which becomes sensible to the touch only in consequence of a particular mode of action, it will follow, that no substance in nature can be _insulated_ with respect to it; but, in whatever place, and for whatever length of time, any substance shall be affected in such a manner as to agitate this fluid, there we shall perceive a production of heat _without limitation_, even though heat itself be no more than the action of a fluid essentially _material_, though invisible to us. Considerations of this kind occurred long ago to the writer of this treatise, when by the nature of his employment it was necessary for him to speculate upon these subjects. It could not then but appear to him that the theory of Dr. Black was far superior to any that had been published. The opinion of those who supposed fire to consist in the vibratory motion of the particles of solid bodies, seemed altogether untenable. It is impossible to explain the phenomena of heat upon ordinary mechanical principles, because, with respect to all terrestrial substances, heat constantly appears as an _agent_, while they are merely _passive_; and no man can explain the nature of a cause from its effect. Thus one of the most obvious effects of heat is _expansion_, or enlargement of bulk, in such bodies as are heated. But if from this fact we infer that the parts of elementary fire are repulsive of one another, our reasoning is certainly erroneous. In like manner, when we are not sensible of heat, we are not authorised to conclude that it is not present; for Dr. Black has demonstrated that it may be present in very great quantity, though indiscoverable either by our senses or by a thermometer.