Sylvius explains the preparation of the vital spirits in the encephalos by distillation, and he finds a great resemblance between their properties and those of spirit of wine. The nerves conduct these spirits to the different parts, and they spread themselves in the substance of the organs to render them sensible. When they insinuate themselves into the glands the addition of the acid of the blood produces a liquid analogous to naphtha, which constitutes the _lymph_. Lymph, then, is a compound of the vital spirit and the acid of the blood. _Milk_ is formed in the mammæ by the afflux of a very mild acid, which gives a white colour to the red humour of the blood. The theory of the natural functions was no less chemical. Even the diseases themselves were explained upon chemical principles. Sylvius first introduced the word _acridity_ to denote a predominance of the chemical elements of the humours, and he looked upon these _acridities_ as the proximate cause of all diseases. But as every thing acrid may be referred to one or other of two classes, acids and alkalies, there are only two great classes of diseases; namely, those proceeding from an _acid acridity_, and those proceeding from an _alkaline_. Sylvius was not altogether ignorant of the constituent parts of the animal humours; but it is obvious, from the account of his opinions just given, that this knowledge was very incomplete; indeed the whole of his chemical science resolves itself into a comparison of the humours of the living body with chemical liquids. Perhaps his notions respecting such of the _gases_, as he had occasion to observe, were somewhat clearer than those of Van Helmont. He called them _halitus_, and takes some notice of their different chemical properties, and states the influence which he supposes them to exert in certain diseases. In the human body he saw nothing but a magna of humours continually in fermentation, distillation, effervescence, or precipitation; and the physician was degraded by him to the rank of a distiller or a brewer. Bile acquires different acridities, when bad food, altered air, or other similar causes act apon the body. It becomes _acid_ or _alkaline_. In the former case it thickens and occasions obstructions; in the latter it excites febrile heat; and the viscid vapours elevated from it are the cause of the cold fit with which fever commences. All acute and continued fevers have their origin in this acridity of the bile. The vicious mixture of the bile with the blood, or its specific acridity, produces _jaundice_, which is far from being always owing to obstructions in the liver. The vicious effervescence of the bile with the pancreatic juice produces almost all other diseases. But all these assertions of Sylvius are unsupported by evidence.