Chemistry

Chemistry may be defined as the science which treats of the composition of substances, and the changes produced in them by physical forces or by interaction with other substances.

The determination of the composition of substances or their analysis involves (1) a qualitative analysis, or the determination of the elements present, and (2) the quantitative analysis, a determination of the relative proportions of the respective components. These two together constitute the department of analytical chemistry. The branch of this department which deals with the analysis of metallic ores, or alloys, etc., is generally designated by a special name - assaying. The ascertainment of the relative quantities of the elements present in a body is not, however, sufficient for the complete knowledge of its composition. The absolute number of the atoms present, and their arrangement in the molecule - constitution - must also be determined. This is frequently only possible by a careful and arduous study of the physical properties of the substance, and the effects of its interaction with other compounds.

The study of the relations between the physical properties of a body and its chemical composition, together with the effects of physical forces, as heat, light, etc., on it, constitutes the branch of physical chemistry or chemical physics, and forms the border which joins, rather than separates, the two kindred sciences.

Until the commencement of the present century almost all the known compounds were those obtained either from minerals, or by appropriate methods from organic material. It was believed that between these two classes of bodies there was an essential difference, and that the latter owed their origin to a "vital force," and could not be prepared synthetically from mineral sources. Owing to this belief, Lemery first in 1675 divided the science into two branches, Organic and Inorganic Chemistry. The artificial preparation of urea by Wohler in 1828 was quickly followed by the synthesis of many other so-called organic bodies, and effectually demonstrated the non-existence of any fundamental distinction between inorganic and organic compounds. All the latter, however, being found to contain carbon, and, as the known compounds of this element are excessively numerous, being greater in number than those of all the other known elements, the divisions are still retained, though the terms have lost their original significations. Organic chemistry may now be defined as the chemistry of the carbon compounds, inorganic being that of the compounds of the other elements. The branch of inorganic chemistry which treats of the composition, etc., of naturally occurring minerals, receives the title of mineralogical chemistry. The study of the chemical nature of substances entering into the constitution of the animal organism, and the chemical changes taking place during the life processes of animals, forms the domain of physiological chemistry. The investigation of the influence of soils, and manures, etc., of different compositions, upon vegetable life, and the chemical principles underlying the art of agriculture, are included in the province of agricultural chemistry. Pharmaceutical chemistry deals with the nature and mode of preparation of the various drugs, ointments, etc., employed for medicinal purposes. The science in its relations to the arts, manufactures, and industrial processes is embraced in the wide titles of technical and applied chemistry. Metallurgy might be regarded as a division of this science, but is generally treated as a separate branch of study. By the term "changes," as employed in chemistry, is understood chemical changes, i.e. alterations in the chemical nature or composition of the substance; in contradistinction to physical changes which only affect the state of the body, the composition remaining unaltered. Thus by the action of heat water becomes converted into steam, but this involves no alteration in the composition, as steam and water are identical in this respect, and on cooling water is again formed. If an electric current, however, be passed through it, water is decomposed and a mixture of two gases is obtained which do not on cooling recombine.

To represent the composition of bodies and the changes they undergo in different reactions, in a concise and simple fashion, chemists adopt a system of chemical nomenclature. That in use at the present time may be briefly described. Each element is represented by a symbol, which is generally the first letter, or characteristic letters, of its name, or in the case of the commoner and longer known elements, its Latin name. Thus oxygen is represented by the symbol 0, silver by the symbol Ag. (Argentum). This symbol is used not only to represent the element qualitatively but also quantitatively, being used to signify a weight of the element proportional to its atomic weight. By the combination of symbols formulae are obtained representing compound substances, the number of such proportional weights being indicated by a small numeral placed at the right of, and below each symbol. Thus the formula ,of water is OH₂, i.e. it consists of 16 parts of oxygen and 2x1 parts of hydrogen by weight. The formula may also be taken as representing the molecule of the compound, and the formula OH₂ would then denote one molecule of water, i.e. one atom of oxygen combined with two atoms of hydrogen. A chemical action is represented by what is known as a chemical equation, in which the formulas of the reacting substances separated by the sign + are placed on the left side, and the formulae of the resulting products on the right side, the two sides being joined by the sign of equality =. It is evidently a quantitative as well as a qualitative relation. Thus the equation

NaOH 4- HC1 = NaCl -f OH₂

states that by the interaction of 40 parts of caustic soda and 36.5 parts of hydrochloric acid, there result as products 585 parts of sodium chloride and 18 parts of water. The law of the indestructibility of matter serves as the basis of all chemical equations, in which the quantity of any particular element must be the same on each side. Frequently symbols are also used to denote groups of atoms which remain unchanged throughout many reactions. This is especially the case in organic chemistry, in order better to represent the constitution of substances, and to simplify the nomenclature. Thus the group of atoms C₂H₅ is represented by Et. and known as ethyl. These groups of atoms receive the name of radicals. As frequently two different substances may possess the same percentage, composition, and same number of atoms, formulae are frequently employed which endeavour to represent the arrangement of the atoms. These are known as constitutional formulae, and are of more frequent use in organic than in inorganic chemistry.

A chemical change, in which two or more substances unite to form a new compound, is termed a chemical combination, for laws governing which see Atomic Theory. The reverse reaction, where one compound is broken up into two or more substances, is called a decomposition, while those reactions, by far the most numerous, which involve the destruction of the reacting bodies with formation of new compounds are known as double decompositions or metatheses. Reactions in which more complex compounds are obtained from simpler are termed synthetical. When the reverse obtains the action is spoken of as analytical.

The word chemistry is derived from the Greek chemeia, the original signification of which, however, is doubtfull. It is, perhaps, however, identical with the Egyptian word Chemi- = "Egypt." In such a case the meaning of chemistry was originally the Egyptian art. The Phoenicians, Egyptians, Greeks, and Romans possessed a crude knowledge of various chemical facts, chiefly metallurgical or technical, but the facts were not collected or correlated, neither did the ancients occupy themselves to any extent in chemical experiment. They were, however, prone to speculation, and from the Aristotelian doctrine of a primordial matter, of which all substances were but modifications, sprang the belief in the transmutation of metals. In this alchemy (q.v.) had its origin, and though the exertions of the alchemists, in their search for the philosopher's stone, were productive of a great increase in the number of chemical facts known, chemistry, as a science, cannot be said to have existed until the time of Boyle (1627-1691). During the early part of the sixteenth century the application of chemistry to medicine received an impetus through the work of Paracelsus (and afterwards of Van Helmont), who endeavoured to fuse medicine and chemistry into one branch of study. With Boyle begins the era of exact experimenting and of researches into the composition of substances. He, amongst other work, first clearly defined the meaning of the term element, and brought forward the law which bears his name. Contemporaneous was Mayow, and shortly afterwards Lemery advanced the science and first drew the distinction, now meaningless, between organic and inorganic chemistry. Becher and Stahl (1660-1734) promoted the further development, the latter advancing the theory of combustion known as the Phlogiston Theory, which was for the next century generally accepted by scientists. Margraff first introduced the microscope as an aid in chemical investigations, and accomplished a large amount of useful work. The latter portion of the eighteenth century stands out as an epoch of brilliant chemical discoveries, chiefly through the exertions of Black, Priestley, Scheele, Lavoisier, and Cavendish. The latter first discovered the composition of water, and demonstrated the balance as the instrument par excellence of the chemist. Priestley, amongst much other work, discovered oxygen (1774), simultaneously found by Scheele, who added chlorine and manganese to the list of known elements. With Lavoisier dawns a new era in the history. His great work was the overthrow of the Phlogiston theory and establishment of the present, theory of combustion and oxidation. The next great step was the enunciation of the Atomic theory by Dalton (1803), which was further developed by the labours of Davy, Gay-Lussac, and Berzelius. The former, also, is illustrious as the discoverer of the metals of the alkalis and the alkaline earths. Gay-Lussac originated the law associated with his name, and Berzelius first determined the atomic weights of a large number of the known elements. Avogadro (1811) brought forward "Avogadro's law," and about 1820 Dulong and Petit discovered the law connecting specific heat and atomic weight; and Mitscherlich enunciated the law of "Isomorphism" (q.v.). About 1861 the application of the spectroscope to chemical analysis by Bunsen and Kirchoff opened up an entirely new, wide, and fruitful field of investigation. From this time on the number of workers is legion. The discovery of the Periodic Law (q.v.) by Newlands (1864) and its further development by Lothar Meyer and Mendeleef must be regarded as the greatest step in modern times.

About 1820 organic chemistry, chiefly through the exertions of Dumas, Liebig, and Wohler, began to make very rapid strides, and a great multiplicity of compounds were soon prepared and their compositions determined. The classification of these compounds was at first very unsatisfactory. Laurent and Gerhardt, by referring all to simpler substances as types, considerably simplified the science and materially advanced its growth. Their work was further developed by Wurtz, Hoffmann, and Williamson. The discovery of benzene in coal-tar by Hoffmann and the preparation of the first aniline dye by Perkin (1856) was the foundation of the coal-tar industry, which, by the subsequent labours and researches of Perkin, Hoffmann, Lauth, Caro, Griers, and others, has extended to such an amazing extent. It is impossible, owing to the marvellous growth, to even briefly notice the later history, besides which the subsequent period is too near the present for us properly to appreciate the value of the various chemical discoveries.