« PreviousContinue »
tricity of small Tension: ELECTROLYSIS ......
Electrolytes, Ions, and Products of Decomposition in General.............
Degree of Decomposition
Place of Decomposition.
Relation between the Quantity of the Current and the Quantity of
Liquid decomposed ......
Influence of the Intensity of the Current on Decomposition
Influence of the Chemical Nature of the Electrolyte ......
Influence of the Temperature and Compression of the Electrolyte
Influence of the Relative Volume of the Electrolyte........
Influence of the Chemical Nature of the Electrodes
Influence exerted by the Surface of the Electrodes .................................
CHEMISTRY is a branch of Natural Science.
Natural Science embraces the whole range of sensible objects, their properties, and the changes to which they are subject, in so far as these changes are not referable to the inward workings of the human mind.
Some branches of Natural Science consider the several classes of bodies in a state of rest according to their situation, magnitude, form, structure and other physical properties: such are Descriptive Astronomy and Geology, Mineralogy, Botany, Zoology, Animal and Vegetable Anatomy. Other branches of Natural Science take into consideration the changes to which the various classes of bodies are subject, without reference to the causes by which these changes are brought about; e.g., Physical Astronomy and Geology, Animal and Vegetable Physiology, Nosology, Therapeutics. Lastly, Physics and Chemistry regard these changes, not according to the classes to which the bodies belong, but according to the several causes by which the changes are produced. All the changes to which the bodies of the universe are subject may be referred to the following causes or Forces.
I. Repulsion: which manifests itself, either in the form of Impene-
without any alteration of their properties excepting as regards
space which they occupy; and this may be subdivided into-
between large masses.
and at immeasurably small distances only.
acting at immeasurably small distances only, and pro
ducing a heterogeneous mass.
bodies of different kinds, acting at immeasurably small
distances only, and producing a homogeneous mass. III. Vital Force, or that peculiar power by which the most important alterations in living organized bodies are produced.
All changes produced by mechanical Attraction belong to the province of Physics; the phenomena resulting from vital force to that of Animal and Vegetable Physiology.
Chemistry relates exclusively to those changes of bodies which are brought about by Affinity. Now since these changes consist—(1) in the combination of dissimilar bodies into a homogeneous mass, and (2) in occasionally resulting separations of dissimilar kinds of matter from a homogeneous mass, it follows that Chemistry may be defined as the science which treats of the combination of dissimilar bodies into homogeneous masses, and of the separation of dissimilar bodies from homogeneous masses.
The range of Chemistry thus marked out is enlarged by the following considerations.
1. By a short account of Cohesion and Adhesion.
2. By the enumeration of the physical properties of the elementary bodies and of the compounds formed by their
mutual affinities, particularly of those which do not come within the province of Mineralogy.
On the other hand, the range of the science is narrowed by the consideration that the relations of the Imponderables need not be treated in all their detail, inasmuch as the study of these bodies forms an important branch of physical science.
Chemistry, regarded as an independent science embracing the whole range of chemical knowledge and regarding the chemical properties of the various kinds of matter without reference to any application of them beyond the bounds of the science itself, is distinguished by the name of Pure, Theoretical or Philosophical Chemistry. And inasmuch as particular branches of it serve to throw light on other sciences, these several branches have received the names of Physical, Mineralogical, Physiological, Medical, Agricultural Chemistry, &c.
Chemistry is an Art as well as a Science. For it teaches not only what combinations of dissimilar bodies into homogeneous masses and what decompositions of these latter are possible; but likewise what rules must be followed, what mechanical means must be adopted to bring about these combinations and decompositions. This is Practical Chemistry. It is subdivided into Synthetical Chemistry the object of which is the union of dissimilar bodies into homogeneous masses; and Analytical Chemistry which teaches how to detect the several constituents of a compound body and estimate their quantities. The art of Assaying or Docimasy, which relates to the detection and estimation of the precious metals in their various compounds, is a branch of analytical chemistry.
Applied Chemistry comprehends the art of preparing the various substances used in the ordinary business of life, in so far as such preparation is based on chemical principles. Its divisions are : Pharmaceutical Chemistry which relates to the preparation of substances used in medicine; and Technical Chemistry which is concerned with arts and manufactures. This latter is again subdivided according to the several products whose preparation it embraces,—and more particularly into the following branches: viz. Metallurgy, or the production of metals: Lithurgy, or the chemistry of minerals: Hyalurgy, or the chemistry of glass-making: Phlogurgy, or the chemistry of combustible bodies: Halurgy, or chemistry of salts, acids and alkalis: Chromurgy, or chemistry of colours: Zymotechny, or chemistry of fermentation, &c.
HISTORICAL SURVEY. In ancient times the progress of chemistry was slower, in modern times it has been more rapid than that of any other science. Up to the end of the seventeenth century, chemical knowledge was confined to a few isolated facts, either relating to metallurgy and other chemical arts or to the preparation of medicines, or such as were accidentally discovered during the fruitless search of the alchemists after the philosopher's stone, the transmutation of metals, &c. The marshalling of these facts into a regular system was begun by Becher and Stahl about the end of the seventeenth, and brought to a more advanced stage by Lavoisier towards the end of the eighteenth century.
Among the nations of antiquity the Egyptians appear to have possessed the greatest amount of chemical knowledge. They prepared sal-ammoniac, soda, common salt, vitriol, glass, enamel, tiles, painted earthenware, several metals and metallic alloys, soap, beer, vinegar, various medicines and pigments, and knew how to fix colours on silk by means of mordants, and to preserve dead bodies from decay. The knowledge of the Egyptians extended itself to other nations, particularly the Jews and Greeks. Whether the Chinese, who have long been acquainted with the preparation of sulphur, nitre, gunpowder, borax, alum, porcelain, verdigris, paper, together with dyeing and the formation of various metallic alloys, are partly indebted to the Egyptians for their knowledge—is a question which must remain undecided.
The Greeks sought to penetrate the secrets of nature less by accurate observation than by speculative enquiry. The assumption of the four elements (fire, air, earth and water) by Anaximander and other Grecian philosophers affords evidence however of a right comprehension of the four principal states in which matter can exist. Less conformable to nature is the opinion adopted by Aristotle and others that all matter is essentially the same, and that the various forms of it are due merely to differences of shape in the ultimate particles. The Romans derived all their chemistry, as indeed all the rest of their knowledge, from the Greeks without making the smallest extension of it. The migration of the northern nations and the overthrow of the Roman empire put a stop to the progress of science in Europe.
On the other hand, from the seventh to the eleventh century, the sciences, and chemistry in particular, obtained protection and cultivation among the Arabs who had spread themselves over Spain and the north of Africa. Their chemical investigations were chiefly directed to the preparation of medicines and the conversion of base into noble metals. They aimed at preparing substances which, like the philosopher's stone, should free the base metal from its imperfections or blemishes and thereby convert it into a noble metal: they believed also that such a substance or one of like nature would restore the sick to health and even render man immortal. Hence they laid the foundation of Alchemy. Geber, one of the earliest chemists, who lived in the eighth century, was acquainted with milk of sulphur, nitric acid, aqua regia, solution of gold, nitrate of silver, corrosive sublimate, red oxide of mercury, the preparation of litharge, &c. Albukasis, towards the end of the twelfth century, described a form of still like that at present used in the distillation of brandy; also the distillation of vinegar and wine. The words Alkali, Alcohol, Aludel, &c., which are still in constant use, originated with the Arabs.
The crusades served to transplant the chemical knowledge and views of the Arabs into Europe, where, agreeably with the mystical tendencies of the age, the theory of transmutation found especial favour; and while it raised up persevering alchemists who employed themselves in experiments laborions as they were fruitless for the purpose of obtaining gold from the baser metals, at the same time gave birth to a host of impostors who turned the credulity of others to their own account. This rage for alchemy continued in Europe from the thirteenth to the seventeenth century, and though it failed in attaining the end proposed, gave rise to the discovery of a mass of facts which, had it not been for this thirst for gold, would in all probability have long remained unknown. Among the most renowned of the alchemists were: Arnold de Villa Nova in the thirteenth century, Raimund Lully in the fourteenth, Basil Valentine