72. REFRACTION OF LIGHT BY LIQUIDS 73. INFLUENCE OF ATOM-LINKAGE ON REFRACTION 105. THE FUNCTION OF THE IONS IN THE PRODUCTION OF ELECTRIC CURRENTS PAGE 183 184 186 189 111. 112. . GULDBERG AND WAAGE'S THEORY OF THE ACTION OF MASS 196 197 113. AVIDITY OF ACIDS 199 114. AVIDITY CALCULATED FOR MOLECULAR WEIGHTS 202 115. RELATION BETWEEN THE AVIDITY AND THE COMPOSITION OF ACIDS P. 32, seven lines from bottom, instead of 'theory of molecular impacts' read theory of starting molecules'; and in the same line, instead of molecular theory' read 'kinetic theory.' § 1. Definition and Province.-Chemistry is a most important branch of natural science. As the human mind is incapable of embracing knowledge in its entirety, it is necessary to divide science into several branches. The sciences may be classified either according to the methods of investigation employed or according to the objects investigated. In the first system we distinguish between descriptive science, sometimes inaptly termed natural history, and natural philosophy, which should in reality be styled natural history. The investigation and description of the various objects as they occur in nature is the problem the descriptive sciences have to deal with, whilst it is the aim of natural philosophy to investigate their genesis and transformations, and to endeavour to discover the cause of these changes. Chemistry 1 belongs to both branches of science. If we examine any natural object, such as a rock, an animal, or a plant, we find, as a rule, that it is composed of many dissimilar parts. The rock is composed of different minerals, the animals and plants are composed of different organs; these, again, are built up from more elementary forms, such as cells. This subdivision cannot be carried on indefinitely we finally arrive at forms of matter which cannot be split up by mechanical means into dissimilar particles. Chemistry is the science which investigates and describes these ultimate constituents, of which The origin of the word 'Chemistry' is not known with certainty. B all natural objects are composed. Chemistry is, therefore, a fundamental portion of all descriptive science. But, on the other hand, chemistry is also one of the explanatory sciences. Almost all the natural phenomena with which we are acquainted are of a complex nature; the eruption of a volcano, an earthquake, a thunderstorm, a fire, the life and growth of animals and plants, and numerous other occurrences, are composed of several distinct phenomena, such as light, heat, sound, electricity, evaporation, and other changes of condition. Natural philosophy treats of these elementary changes into which natural phenomena resolve themselves. Natural philosophy embraces physics and chemistry. It is the aim of physics to investigate and explain those elementary changes which affect the properties of bodies without altering their material composition. Chemistry deals with the changes which affect the material nature of the substance. Chemistry, then, is the science which treats of matter and its changes. § 2. Characteristics of Chemical Change.-Numerous material changes in natural objects are continually taking place, such as the formation of organic compounds in plants, the various changes which animal and vegetable bodies undergo either in nature or by the agency of man; for example, fermentation, putrefaction, combustion, the extraction of metals from their ores, the preparation of food, drugs, dyes, and innumerable other materials. These changes in the composition of bodies have been taking place from time immemorial before the eyes of men, generally, indeed, at man's desire; but in spite of this, for thousands of years they have been involved in obscurity, and even at the present time they remain incomprehensible to the majority even of educated people. Although chemical changes are continually taking place everywhere, the cause of these changes is difficult to recognise. This peculiarity of chemical phenomena is an inherent result of their nature. By exposing one or more substances to certain conditions, an entire change in their nature is effected. This may be brought about by exposing the substance to the action of heat, light, or percussion; indeed, in some cases a chemical change takes place when the substance does not appear to have been subjected to any kind of external influence. Sulphur burns, and leaves in its place a METHOD OF INVESTIGATION 3 pungent-smelling gas. Coal is heated, and produces coal gas. Ores heated with charcoal yield metals. Iron rusts in the air. Molten lead changes into litharge, a dull powder, which is reduced to lead when heated with charcoal. These changes, and thousands of similar transformations, appear to be of a mysterious and marvellous character. Compare them with such phenomena as the movement of a falling body, the reflection or refraction of a ray of light, the heating and cooling of a body, the action of one magnet on another, &c., and we see that it is not very difficult to study the whole course of most physical phenomena; whereas in the case of chemical changes the beginning of a reaction is, as a rule, immediately followed by its conclusion, so that it is impossible to perceive the intermediate stages. This is the reason why chemistry remained for thousands of years a mere collection of recipes and mystic formulæ, in spite of the labour which had been devoted to its advancement. This explains, also, how it was possible for chemistry to exist for centuries in a condition hardly worthy of the name of a science, side by side with a highly developed state of physics. § 3. Method of Investigation.—The high state of development which science has attained at the present day has been gained by a logical application of the method of induction. The numerous isolated facts presented to our observation are so classified that allied and analogous facts are arranged together for the purpose of comparison. The laws and rules resulting from this comparison are gradually expanded and generalised, or, if necessary, more sharply defined, and their application limited. The knowledge of such laws does not satisfy the human mind-it desires to learn the reason, the cause of the existence of these laws. Now, this knowledge cannot be gained from observation. It is only attained by an effort of our intellect, which solves the connection between the phenomena under observation and the causes which produce and modify them. The knowledge of the causal connection of phenomena is consequently subjective, and it always remains an open question to what extent it is co-ordinated to the objective world. The investigation of this point is a second problem for science to |