The Unknown.—­There is nothing more difficult than a good definition, for it is scarcely possible to express in a few words the abstracted view of an infinite variety of facts.  Dr. Black has defined chemistry to be that science which treats of the changes produced in bodies by motions of their ultimate particles or atoms, but this definition is hypothetical, for the ultimate particles or atoms are mere creations of the imagination.  I will give you a definition, which will have the merit of novelty and which is probably general in its application.  Chemistry relates to those operations by which the intimate nature of bodies is changed, or by which they acquire new properties.  This definition will not only apply to the effects of mixture, but to the phenomena of electricity, and, in short, to all the changes which do not merely depend upon the motion or division of masses of matter.  However difficult it may have been to have given you a definition of chemistry, it is still more difficult to give you a detail of all the qualities necessary for a chemical philosopher.  I will not name as many as Athenaeus has named for a cook, who, he says, ought to be a mathematician, a theoretical musician, a natural philosopher, a natural historian, &c., though you had a disposition just now to make chemistry merely subservient to the uses of the kitchen.  But I will seriously mention some of the studies fundamental to the higher departments of this science; a man may be a good practical chemist perhaps without possessing them, but he never can become a great chemical philosopher.  The person who wishes to understand the higher departments of chemistry, or to pursue them in their most interesting relations to the economy of Nature, ought to be well-grounded in elementary mathematics; he will oftener have to refer to arithmetic than algebra, and to algebra than to geometry.  But all these sciences lend their aid to chemistry; arithmetic, in determining the proportions of analytical results and the relative weights of the elements of bodies; algebra, in ascertaining the laws of the pressure of elastic fluids, the force of vapour as dependent upon temperature, and the effects of masses and surfaces on the communication and radiation of heat; the applications of geometry are principally limited to the determination of the crystalline forms of bodies, which constitute the most important type of their nature, and often offer useful hints for analytical researches respecting their composition.  The first principles of natural philosophy or general physics ought not to be entirely unknown to the chemist.  As the most active agents are fluids, elastic fluids, heat, light, and electricity, he ought to have a general knowledge of mechanics, hydrodynamics, pneumatics, optics, and electricity.  Latin and Greek among the dead and French among the modern languages are necessary, and, as the most important after French, German and Italian.  In natural history and in