from their colours; for hot, cold, dry, and humid substances are found of every colour. And yet from the colour of every kind of seeds, roots, or juices it is possible to derive a certain indication of their temperament. For example, onions, squills, and wine, the whiter they are, are the less hot; but such as are of a yellowish and intermediate colour are hotter. And wheat, vetches, and kidney-beans, and chick-peas, the root of iris, that of kingspear, and many others, are similarly affected. In each genus, for the most part, such things as are gold coloured, red, and of a bright yellow, are hotter than the white, so that if any conjecture can be formed therefrom of the powers of medicines, it is so far well. It is best then, as has been often said and demonstrated, to determine the powers of each by exact experiment, for by this you cannot be deceived; but before ascertaining their powers by experiment, the taste will give many indications, in which it will be assisted in a small degree by the smell.” (_Præfatio in Aëtium._) For a fuller account of the subject, the reader is referred to the first five books of Galen’s work ‘On the Powers of Simples;’ to the first tractate of the Second Book of Avicenna; and to the introductory part of Serapion’s work ‘On Simples.’ A useful abstract of the ancient opinions is given in the small tract of Albengnefit. The nature of the tastes is ingeniously discussed in the ‘Timæus’ of Plato, and by Theophrastus (de Causis Plantarum, vi.) Alkhendi’s theory of the action of compound medicines appears to be ingenious; but it is complex and difficult to explain, being founded upon the principles of geometrical properties and musical harmony. The ‘Chemia’ of Geber contains a very interesting abstract of the knowledge possessed by the ancients regarding the recondite nature of substances, that is to say, on alchemy, but supplies little or no information on the Materia Medica or Pharmacy.