is painfully conscious that any description of experiments must necessarily fall far short of giving that force of impression which the phenomena of Nature produce when they speak for themselves, and, in weighing the arguments presented, he must beg his readers to make allowances for this fact. CAMBRIDGE, September 6, 1873. INTRODUCTION. In most works on chemistry this subject is defined as the science which treats of the composition of bodies; and it is made the chief object to present the scheme of the chemical elements, and to show that, by combining these elements, the innumerable products of nature and the arts may be prepared; and, although the fundamental laws of the science may be fully illustrated, the discussion of these general principles is made a subordinate feature of the work. In the larger treatises, which must consist chiefly of descriptions of substances and processes, this method of treatment is both natural and necessary. But the same plan is almost invariably adopted in the elementary text-books, which are made for the most part compilations of facts, and differ from the larger works chiefly in the brevity and consequent incompleteness of their descriptions. To the great mass of learners the study of these text-books is uninteresting and profitless; for, before the student is made familiar, through long laboratory practice, with the materials and processes described, such a book is little more to him than a catalogue of names to which he attaches no signifi cance. While, however, the facts of chemistry have multiplied to an extent that renders it impossible to present them, even briefly, in a volume of moderate size, the general principles of the science have been so developed that they now form an important body of scientific truths, which may be studied to advantage by themselves, before attempting to grasp the great scheme which the composition of material nature presents. On this plan the present work has been written. The aim has been to develop the general principles of chemistry in a systematic order, and only so far to describe substances and processes as seemed necessary to illustrate these principles. Chemistry is defined as the science which treats of those phenomena of nature that involve a change of substance, and such phenomena are defined as chemical processes. It is shown that a chemical process always consists in the change of certain substances called the factors into certain other substances called the products, and that the first object of chemical investigation is to determine, in regard to each chemical process, what are the factors and what are the products. It is further shown that every chemical process obeys three fundamental laws: 1. That the sum of the weights of the products equals the sum of the weights of the factors. 2. That the relative weights of the several factors and products bear to each other a definite ratio. 3. That, if the factors or products are aëriform, the volumes of such vapors or gases are very simply related. These laws are called respectively The Law of Conservation of Mass; It will be noticed that the fundamental laws of chemistry, thus enunciated, are facts capable of simple experimental illustration, and involve no hypothesis whatsoever. At this point, however, the molecular theory, by which these laws are explained and shown to be related to a system of science, is introduced. The way has already been prepared by stating the general principles of the kinetic theory of gases, by which molecules are defined, and their relative weights established; involving the well-known laws of Mariotte, of Charles, and of Avogadro. It is next made to appear that the molecular weights deduced from the kinetic theory are very simply related to the definite proportions observed in chemical processes; and thus we are led to the chemical as distinguished from the physical conception of the molecule, and it is shown how greatly the coincidence between the chemical and the physical results confirms the molecular theory. The molecule having been further defined as the smallest mass in which the qualities of a substance inhere, it is made clear that in all chemical processes the action must be referred to the molecules of which the several masses of the factors and products are aggregates. Thus far nothing has been said about the composition of matter; but it is now shown that the study of chemical processes requires us to admit that in some cases the material of a product was formerly a part of the material of a factor, while in other cases the material of two or more factors has united to form the material of a single product. Hence arise necessarily our conceptions of decomposition or composition, of analysis or synthesis; and we thus easily reach the further conception of a class of substances, which, while capable of synthesis, are incapable of analysis. These are the elementary substances; and although at this stage the complex processes of chemical analysis can not be fully explained, yet the general principles may be made intelligible, and the method of expressing the percentage composition of chemical compounds clearly stated. From this stage in the development of our chemical philosophy we take the next important step without difficulty. Since the qualities of a substance inhere in its molecules, the composition of the molecule must be the same as the composition of the substance; and the percentage of any element, found from an analysis of a mass of the substance, is the percentage of that element in the molecule itself. These elementary parts of the molecules are the atoms of chemistry, and we thus reach not only a conception of the smallest particles into which matter has been subdivided, but also attain to a knowledge of the general method by which the atomic weights have been established. When the conceptions which modern chemistry connects with the words molecule and atom have been realized by the student, the meaning of the symbolical language of the science is made intelligible with only a few words of explanation. The simple symbols stand for the atoms, with their invariable relative weights; molecules are represented by writing together the symbols of the atoms of which they consist, indicating the number of atoms of each kind by a subscript Arabic numeral; and these molecular formulæ indicate not only the molecular weight, but all such formulæ also represent equal gas-volumes. Lastly, chemical processes are represented by writing the formulæ of the molecules of the factors as the first member, and those of |