vicissitudes of the atmosphere, which we are made to believe were the causes of that change. Another quotation is made from the same author in which a warm, dry, stagnating air is _conjectured_ to have been the cause of _diseases_; but he does not even quote Dr. Rush saying that it was the cause of yellow fever, much less of all epidemic diseases. Besides, to say that any thing is occasioned by a _state_, or _vicissitude_ of the atmosphere, is such a vague mode of expression, that it must either mean nothing, or be contradictory to itself. A _state_ of the atmosphere we must suppose to mean that it continues for some time either to be wet or dry; a _vicissitude_, when it changes from one to the other. If an epidemic then is produced by a _state_, it cannot also be produced by a _vicissitude_, of the atmosphere: or, if some epidemics are produced by states, and others by vicissitudes, we ought to be informed which produce one kind, and which another. But throughout the whole of this dissertation we have neither distinctness nor regularity, nor indeed any thing but assertion, supported only by an imaginary theory. Dismissing at length therefore these conjectural theories, let us endeavour to deduce from certain and undoubted facts the connexion between the state of the body, and the operations upon it of other causes, invisible indeed to our eyes, but discoverable by our rational faculties, and in some measure capable of being made the objects of our senses also. 1. From the account given of the structure of the human body, it undeniably follows, and has already been observed, that all parts of it are so connected together, that none can suffer any very grievous injury without affecting all the rest. 2. The life of man depends immediately on the air. From this element the _blood_ receives heat and a vital spirit diffusing itself from the blood along the nerves, and thence expended in the operations of life and sensation. 3. From undoubted experiments[89] it appears, that this vital spirit possesses in a great degree the properties of electricity, insomuch that many suppose them to be the same. This is indeed denied by the celebrated anatomist, Dr. Monro, but he allows that the nervous fluid is similar to electricity, and it is certain that the electrical fluid can affect it in such a manner that we may reasonably believe them to be the same. [89] Those of Galvani and others on _animal electricity_.