90. MIXTURE OF GASES. DIFFUSION. EFFUSION. TRANSPIRATION PREPARATORY ΤΟ CHEMICAL 111. 112. GULDBERG AND WAAGE'S THEORY OF THE ACTION OF MASS. 206 116. CONNECTION BETWEEN AVIDITY AND OTHER PROPERTIES OF THE ACIDS 214 be § 1. Definition and Province.-Chemistry is a most important branch of natural science. As the mind is incapable of embracing knowledge in its entirety, it is necessary to divide science into several branches. The sciences may 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 form the problem with which the descriptive sciences deal, 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 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 of different organs; these, again, are built up from more elementary forms, such as cells, &c. But this subdivision cannot be carried on indefinitely, for 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 'The origin of the word 'chemistry' is not known with certainty. B constituents, of which 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 the combined effect 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 such as are produced 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 occurring 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 pungent-smelling gas. Coal is heated, and produces coal gas, ores are heated with charcoal and yield metals. Iron rusts in the air, molten lead changes into a dull powder called litharge, which is reduced to lead when heated with charcoal. These changes, and thousands of similar transformations, were regarded and treated as mysteries. Compare these changes 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 entire course of such 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. For this reason 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 chemistry should have existed 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 to 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 alone, but by an effort of our intellect the connection between the phenomena under observation and the causes which produce and modify them are solved. The knowledge of the causal connection of phenomena is |