of oxygen gas, that universally-diffused element with which all other elementary substances combine, and of whose compounds almost the whole of terrestrial Nature consists. Lavoisier inferred that oxygen must be the chemical centre in the scheme of Nature, and he therefore made its compounds the basis of a new classification, which, subsequently, Berzelius greatly systematized and improved. In this classification the compounds of the elements with oxygen were divided into two classes: Those which, when dissolved in water— combined with it we should now say-gave an acid reaction, were called acids; while those which, under the same circumstances, gave an alkaline reaction, were called bases. It was known then, as well as now, that these reactions could not be obtained without the presence of water, and that the larger part of the oxides, being insoluble in water, do not give the reactions at all; but, then it was supposed that the water acted only through virtue of its solvent power, that some other solvent would do as well, and that the insoluble oxides would give the same reactions if only an appropriate solvent could be found. Hence, these insoluble oxides were classed with the acids or bases, according as they combined most readily with bases or acids respectively. The insoluble SiO, combined with soda, like the soluble SO,, and hence was classed with it as an acid. So the insoluble FeO combined with sulphuric acid, like the soluble CaO, and hence was classed with the last as a base. Again, the neutralizing of an acid by an alkali had all the appearance of direct combination, and, in all these processes, the acid oxide was assumed to unite with the metallic, or basic, oxide to form what was called a salt. The presence of the water, and the fact that it facilitated the chemical change, were not

ignored, but, as before, it was supposed to act in virtue of its solvent power, and a sufficient number of cases were known where the same compounds could be obtained with and without the aid of water to render this opinion not improbable. Take a single example: Phosphate of lime may be made in two ways: first, by adding to a solution of lime in water a solution of phosphoric acid :

(3Ca=02-H2 + 2H3=0,=PO + Aq.) =

=

Ca2Oo"(PO)2 + (6H-O-H + Aq.).

Secondly, by uniting lime, the oxide of the metal calcium, directly to P2O5, the oxide obtained by burning phosphorus (page 185):

In the last reaction there is no water present, and the first reaction was formerly supposed to be a case of similar direct union between CaO and P2O5, the only difference being that the two oxides were in solution:

59

3(CaO,H2O) + 3H2O,P2O5 = 3CaO,P2O5 + 6H2O. Accordingly, it was customary to write the symbols as in this last reaction, separating the acid from the basic oxide by a comma. Here are a few other exam

As expounded and illustrated by Berzelius, the dualistic theory had the charm of great simplicity, and was greatly strengthened by the electro-chemical facts which he brought forward in its support. The division of the elementary substances into electro-positive and electro-negative elements corresponded very closely to

1 To avoid confusion, all our symbols stand for the new atomic weights, and this must be remembered in comparing these formula with those in the old books.

T

the distinction between metals and metalloids. Bases were compounds of electro-positive elements with oxygen; and acids, on the other hand, the oxides of electronegative elements. Again, among these binary compounds the basic oxides were electro-positive, and the acid oxides electro-negative. Moreover, the wider apart in their electrical relations, the stronger was seen to be the tendency of both the elements and of their oxides to combine, and, just as the metals united to metalloids, so bases united with acids. Thus was formed the class of ternary compounds, called, as above, salts.' Among these, also, could be distinguished a similar opposition of relations, although less marked, to that between bases and acids, and, from the union of two salts, resulted the class of quaternary compounds, or double salts. In this way the theory advanced from elementary substances to the most complex compounds through the successive gradations of binaries, ternaries, and quaternaries; the elements or compounds only combining with substances of the same order, two and two together, like two magnetic poles, or two electrified bodies.

This dualistic theory was certainly a most admirable system, and served the purposes of a rapidly-grow

1 The word salt was used in chemistry very early to describe any saline substance resembling externally common salt; but, under the dualistic system, the term came to be applied to that class of compounds which were supposed to be formed by the union of basic and acid oxides, as described above. Absurdly enough, however, common salt was thus ruled out of the very class of compounds of which it had previously been regarded as the type, and Berzelius, in his electro-chemical classification, made a distinct family of those substances which resemble common salt in their chemical composition, and called it the haloids. But this name -bodies resembling salt-only rendered the anomaly the more glaring, and it was always a blemish on the dualistic system. In the modern chemistry, the word salt, although still used as a descriptive name, bas no technical meaning.

WHEREIN THE DUALISTIC THEORY FAILED.

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ing science for more than half a century. We now feel assured that the old theory undervalued essential circumstances, and misinterpreted important facts. We maintain that hydrogen is an essential, not an accidental constituent of all acids and all alkalies, and that, when the alkali is neutralized by the acid, the reaction consists in the replacement of this hydrogen, and not in the direct union of two oxides. Nevertheless, given the old facts, the old theory was logical and consistent, and it is no longer tenable, not because the old facts have changed, but simply because a whole new order of facts has been discovered by which the old facts must be interpreted. During the last twenty-five years there has been discovered a great mass of truths, connected chiefly with the compounds of carbon, in what was formerly called the domain of organic chemistry, and this is to-day the most prominent and attractive portion of our science. Moreover, the law of Avogadro and the doctrine of quantivalence are two new principles which our modern science has added to the old chemistry, and these principles have supplanted the dualistic theory. Let us not, however, undervalue the old theory. It was an important stage in the progress of science, and a noble product of human thought. Theories are means, not ends; but they are the appointed means by which man may raise himself above the low level of merely sensuous knowledge to heights where his intellectual eye ranges over a boundless prospect which it is the special privilege of the student to behold. What though his vision be not always clear, and his imagination fill the twilight with deceptive shapes which vanish as the light of knowledge dawns; yet, to have enjoyed the intellectual elevation, is reward enough for all his devotion and all his toil.

LECTURE XIII.

ISOMERISM, AND THE SYNTHESIS OF ORGANIC COMPOUNDS.

HAVING, in the previous lectures of this course, made you familiar with the conception that the molecules of every substance have a definite atomic structure, which is a legitimate object of scientific investigation, I endeavored in my last lecture to illustrate, by numerous examples, the mode now generally employed in chemistry of exhibiting this structure by means of what are called structural formulæ, and, during the whole course of these lectures, it has been a chief object to develop the fundamental principles on which these formulæ are based, in order that, having reached this stage, you might be able to see for yourselves that they were legitimately deduced from the facts of observation. I have freely admitted that they were the expression of theoretical conceptions which we could not for a moment believe were realized in Nature in the concrete forms, which our diagrams embody. But I have claimed that they were at present our only mode of representing to the mind a large and important class of facts, and were to be valued as the first glimpses of some great, general truth, toward which they direct our investigation. Theories are the only lights with which we can penetrate the