of the elements, in a molecule, we get the molecular weight. Thus for water, H,O, it is 18; for carbonic dioxide, CO,, it is 44. The symbols and atomic numbers of all the elements are given in a table in the Appendix.

§ 2. Progress of Chemical Theory.

237. Earlier Views.-The progress of chemistry has consisted in the advance of theory, that is, in an ever-widening view of facts, and a deeper insight into their relations. The most ancient theories of material things referred them to some essential principle, as air, water, or fire; and then, later, these ideas were combined. The objects of Nature were held to be formed of various commixtures of four elements, fire, air, earth, and water; and for many centuries the properties and changes of all substances, animate and inanimate, were explained on this hypothesis. In the seventeenth and eighteenth centuries, alchemy, the old mystical pursuit of the art of gold-making, gradually grew into a rough science of experiment by which much became known of the qualities of different kinds of matter. For a hundred years the explanation of chemical changes was given by the theory of phlogiston. This was held to be a kind of subtile matter, present in all combustible bodies, and absent in all incombustible bodies, and which caused combustion-changes by its escape. The doctrine is now regarded as a very crude one, but it contained truth, and was of great service, in its time. A chemical belief that the discoverer of oxygen, Dr. Priestley, held to the day of his death, could certainly not have been an absurdity. Prof. Cooke has the following excellent remarks on this early theory: "That it was not absurd a single consideration will show. Translate the word phlogiston, energy, and in Stahl's work on chemistry and physics, of 1731, put energy where he wrote phlogiston, and you will find there the germs of our great modern doctrine of conservation of energy-one of the noblest products of human thought. It

was not a mere fanciful speculation which ruled the scientific thought of Europe for a century and a half. It was a really grand generalization; but the generalization was given to the world clothed in such a material garb that it has required two centuries to unwrap the truth."

238. The Binary Theory; Dualism.-With the abandonment of phlogiston as a ruling principle of chemical change the conception of affinity came forward, and chemical effects began to be referred to inherent attractions among different kinds of matter. At the epoch of Lavoisier, affinity was thought of simply as a coupling force. Combination and decomposition were supposed to take place directly among bodies in pairs; elements uniting with elements to form binary compounds, and these uniting again by twos to form double binary or ternary compounds; and, when these were made to act on each other, the reaction was represented as a double decomposition. This was known as the dual theory, and was commended for its simplicity and strongly confirmed both by the beautiful nomenclature which was adapted to it, and by the atomic theory which followed soon after. Powerful aid was also subsequently lent to it by electro-chemistry. Compounds were resolved into pairs by galvanic decomposition, and their elements were supposed to be in opposite electrical states, and to be united by polar forces. In this system the controlling idea was the properties of the elements, which were supposed to give character to compounds, and the main question was, What bodies does a substance yield upon analysis? The properties of compounds were referred to the presence of predominating constituents, and hence oxygen was named as the acid-former, and hydrogen as the water-former. The question as to how the constituents of a compound were grouped was hardly raised; yet it now turns out to be a question of very great importance.

239. Unitary or Substitution Theory.-But, as chemical changes were more closely studied, it was increasingly felt

that dualism, or mere splitting and pairing, gave a totally insufficient account of them. There was a truth in this idea, but it was not the whole truth. The conception of atomic groupings in a molecule, and of the molecule as having a unitary constitution, gradually rose into clearness. It was found that the changes that take place among chemical compounds were rather of the nature of replacements and substitutions, which left the structure of the molecule intact. The constitution of the molecule, therefore, became the main object of investigation. It was found, moreover, that the most opposite elements could replace each other in a group without altering its chemical character. Chlorine, a powerful electro-negative element, could be substituted for hydrogen, a strong electro-positive element, in a compound, without changing its characteristic properties.. Chemical compounds, instead of being likened to magnets, with a twofold attraction of opposite poles, were now. likened rather to crystals whose angles and edges may be replaced by new matter, the form being maintained.

240. Theory of Chemical Types.-The unitary theory attained fuller expression in the theory of types, in which molecular structure first became a basis of classification. Most chemical changes were viewed as replacements, which conformed to a few general modes. As the stones of an edifice may be successively exchanged, leaving the style of architecture undisturbed, so atoms may replace atoms, leav-. ing the types of molecular structure unaltered. This important idea was at the basis of the theory. Gerhardt proposed four such general types or patterns, taking hydrogen, hydric chloride, water, and ammonia, as representative bodies, and classing with them all substances which exhibit analogous reactions. But the exceptional compounds were so numerous that the system was held to be inadequate for classification, though invaluable as a transition-step to something broader and more satisfactory.

§ 3. Theory of Atomicity and Quantivalence.

241. Variable Combining Capacity.-The general theory of chemistry now adopted is the outgrowth of preceding theories, and embodies the truths they have severally attained. But it adds an important principle which throws further light upon chemical operations, and serves to organize into a better system the later facts and ideas of the science. The notion of equality between combining elements, and of equivalence among their atoms, has long been fundamental in chemistry. When the substitution theory arose it was still maintained that the replacements werę, atom for atom. But it is now recognized that the replacing power of different kinds of atoms is unequal, through a very considerable range. The idea of variable combining capacity of atoms and molecules has been worked out with great clearness, and is the distinctive feature of what is now known as the New Chemistry.

242. Atomicity.-While certain kinds of atoms interchange with each other as true equivalents, atom for atom, it is found that in other cases it takes two, three, or half a dozen atoms of one kind to equal one of another kind in combining power. To determine these degrees of equivalence of different bodies, we have but to take some one, which will serve as a measure of comparison between them. Hydrogen answers this purpose. It unites with chlorine, atom to atom, forming the molecule of hydric chloride, HCI; but oxygen cannot combine with hydrogen in this way; it must take two hydrogen-atoms, forming the molecule of water, H,O. Nitrogen again takes three atoms of hydrogen, forming the molecule of ammonia, HI,N; and carbon behaves still differently, demanding four hydrogen-atoms as in the molecule of marsh-gas, H1C. We have, therefore, the four following molecular constructions:

which vary in a regular numerical order.

This may

seem to be accidental, but it is not so, for, if we take chlorine instead of hydrogen as a measure, we shall get similar results, as follows:

Now this is not something that merely happens among a few selected substances; it illustrates a law that has been traced through the whole chemical field. It is obvious that in the first four groupings, the elements chlorine, oxygen, nitrogen, and carbon, can no longer be regarded as equivalents of each other; nor are the sodium, mercury, antimony, carbon, and phosphorus, of the second group, equivalents of each other. Each element seems to have its own atomic capacity. Hydrogen, sodium, and chlorine, go together in ones; oxygen and mercury take hydrogen and chlorine by twos; nitrogen and antimony take them by threes; carbon takes both by fours; and phosphorus takes its chlorine in fives. This varying atomic capacity is called atomicity, and the powers of the different elements in this respect are known as their atomicities.

243. Quantivalence, and its Expressions. To these chemical relations the general term quantivalence has also been applied; and different modes are employed to indicate the several atomicities of the different elements. They are as follows: bodies whose atomic capacity is

Bodies with a higher atomic capacity than one are said to be polyatomic or multivalent. Hydrogen, in the single