In addition to these elements there are sev eral others which are as yet incompletely known. Among them, polonium, actinium and ionitun may be named. New elements are not infrequently discovered, and argon, helium, neon, xenon, krypton and radium have all been brought to light within recent years.
With the help of the elementary symbols, chemical formulae can be constructed, and these are of great help in chemical calculations and reasoning. Some of the symbols are initial let ters only, as H, 0, N, C, for hydrogen, oxygen, nitrogen and carbon; others are formed of two letters, like Ca for calcium and Zn for zinc. Still others are derived from the Latin names of the metals, such as Ag from argentum, sil ver, Fe from ferrum, iron, etc. The formula of a compound is made by writing the proper symbols in juxtaposition, so that NO means a compound of nitrogen and oxygen, HI a com pound of hydrogen and iodine, and so on. When two elements form more than one com pound, these are distinguished by subscript numerals, as in the examples PC12 and PCI5. In these substances one atom of phosphorus unites with three and five atoms of chlorine respectively. In every case the symbol of an element means one atom of the element, and therefore that relative quantity of it which is indicated by its atomic weight. In water, HSO, two atoms of hydrogen weighing two units combine with one atom of oxygen weighing 16 units; and thus the formula tells us in shorthand that the compound contains two parts of one element to 16 of the other. No matter how complex a formula may be, this simple rule invariably applies, and by its means the composition of the substance represented can be calculated. The symbols, moreover, can be combined into equations, from which we may compute the outcome of a given chemical process. This subject, however, is much too abstruse for discussion here. Its details are developed logically from the atomic theory.
To the philosophers who preceded Dalton an atom was the smallest particle of any sub stance which could possibly exist. Thus water might be subdivided and subdivided until, in theory, a limit was attained, and an atom of water was the result. By chemical means a new order of divisibility had now to be recog nized, and the supposed eatoms of water was itself found to be complex and separable into still smaller particles of oxygen and hydrogen. The latter are the atoms of the chemist; the former clusters of atoms are known as mole cules. This distinction was not developed im mediately; its full recognition came slowly, and it derived its importance from certain laws relative to gases which were discovered, partly by Boyle, partly by Gay Lussac and generalized in 1811 by Avogadro. Avogadro's law may be concisely stated as follows: Equal volumes of gases under like conditions of temperature and pressure contain equal numbers of molecules. A molecule is now defined as the smallest particle of any substance which can separately exist; an atom as the smallest particle which can take part in chemical change. Another
law, discovered by Dulong and Petit in 1819, was also highly important, for it was shown that the specific heat of an element was in versely proportional to its atomic weight. In other words, the elementary atoms have all the same capacity for heat, and this rule gave us an important check in ascertaining the true weights of the atoms. The law of Avogadro and the law of Dulong and Petit were brought together by Cannizzaro in 1858, and the system of atomic weights and chemical formulae now in use, which differ in certain essential particu lars from those that were first adopted, was the result of the combination.
In the early days of scientific chemistry the science was divided into two great sections, organic and inorganic, the one dealing with animal and vegetable products, the products of life, and the latter with substances derived from the mineral kingdom. Inorganic chemis try, the chemistry of the metals, the earths, the commoner oxides, bases and salts, • was for the time being the simpler, and during the first half of the 19th century it received the lion's share of attention. At first it was supposed that the two fields of research were absolutely distinct, and that no organic compound could be derived by artificial means from inorganic sources. This belief was overthrown by Woeh ler, in 1827, who showed that urea, an organic body, was easily prepared from inorganic am monium cyanate, and since then a vast number of organic syntheses have been effected. Curi ously enough, urea and ammonium cyanate, al though very different substances, have the same percentage composition, containing the same elements in exactly the same proportions. Later it was found that isomerism, as this semi identity is called, was exceedingly common, and here was a noteworthy phenomenon which called for explanation. Different molecules could be constructed from the same set of atoms, and the simplest interpretation of -the fact was that the latter were differently ar ranged. Out of the same bricks different houses may be erected. The conception of chemical structure, therefore, came into being. What is the arrangement or grouping of the atoms within any given molecule? The earlier attempts at the solution of this problem have now only historical interest, and their consideration is of value to professional students of chemistry alone. If we were to trace the successive stages we should find evi dence of a systematic growth in chemical the ory; but definiteness was hardly possible until after Cannizzaro had established the true sys tem of atomic weights, and formulae had been adjusted in accordance with them. Between 1850 and 1860, however, a new property of the atoms began to be recognized, and this, taken in connection with the formulae based on Avoga dro's law, was the key to the problem. The theory of valence, which is the expression of the newly-discovered property, is as follows: Every atom, as regards its union with other atoms, has a certain atom-fixing power, which is known as its valency, or valence.