The method of scientific investigation is nothing but tile expression of the necessary mode of working of the human mind: it is simply the mode by which all phenomena are reasoned about and given precise and exact explanations. The difference between the operations and methods of a baker weighing out his goods in common scales, and the operations of a chemist by means of his balance is not that the scales in the one case, and the balance in 'the other, differ in the principles of their construction or manner of working; but that the latter is a much finer apparatus and of course much more accurate in its measurement than the former. Probably there is not one here who has not in the course of the day had occasion to set in motion of a complex train of reasoning, of the very same kind, though differing in degree, as that which a scientific man goes through in tracing the causes of natural phenomena.