In placing these lectures before a wider section of the public, I consider it essential to indicate the point of view Ivom which they have been prepared. I regard them as an attempt to follow the development of our ideas of to-day from those that were formerly current. Hence I have only joue back as far as Lavoisier ; because our science assumed a new aspect in his hands, and because it may be held that, as regards development, we are still passing through the upoch inaugurated by him.