452. The majority of chemists reduce the number of types to three, by regarding the hydrogen and hydrochloric acid types as one; other chemists add to the hydrochloric acid, water, and ammonia types, a fourth, which represents the tetratomic bodies,-marsh gas, CH,, is selected as the type of this group.

453. In the next four chapters the principal organic compounds are classified under their different types; the hydrogen type has been retained, and a different order has been followed in the introduction of the types, in order to render the subject as simple and systematic as possible to the student. The new atomic weights, C=12, 0 = 16, are employed constantly in these chapters; they are not distinguished therefore, as in the former part of the work, by the dark line through the symbol.

NOTE. (See page 221.)-Wurtz, in noticing Hunt's claim, remarks, "In the history of the science, Griffin should be noticed as having published similar notions, long before Laurent and Hunt. For all this, neither can Laurent or Griffin, any more than Hunt, pass as the author of the theory of types, and I think this is just. Those who discover the facts which give prominence to an idea, and who, thanks to those facts, introduce the idea into science, and who render it fruitful,-these are the true discoverers. Now, the discoverers of the mixed ethers, of the organic anhydrides, and, I may add, of the compound ammonias, are they who have really brought to light the molecular types which characterise modern chemistry. This is why Williamson and Gerhardt have general credit as the authors of this idea. If they are cited in preference to their predecessors, it is in virtue of facts, and of that sort of supremacy which discoveries, properly so called, exert over pure speculation."

253

СНАРТЕER VII.

HYDROGEN TYPE.

454. The substances treated under the simple and complex molecules of hydrogen are subdivided into two groups, the Positive and Negative.

SIMPLE MOLECULE.

HH.

POSITIVE GROUP.

455. This group embraces-(1) The hydrides of metals and metals proper. (2) The hydrides of the alcohol radicals and the alcohol metals, pure and mixed.

(1.) HYDRIDES OF METALS (PRIMARY DERIVATIVES).

METALS PROPER (SECONDARY DERIVATIVES).

456. Primary derivatives.-Very few hydrides in mineral chemistry are known. One only at present is known, which is formed on the simple molecule; this one is the subhydride of copper, Cu,H, in which two atoms of copper replace one of hydrogen. This hydride speedily oxidizes by exposure to the air, protoxide of copper and water being formed.

457. Secondary derivatives.-The majority of the metals can replace hydrogen atom for atom; that is to say, they are monatomic. The following cannot replace hydrogen in any other proportion; that is to say, they are, in all their combinations, monatomic:—

Na, Na; K, K; L, L; Ca, Ca; Sn, Sn; Ba, Ba; Mg, Mg; Zn, Zn; Cd, Cd.

458. A few of the metals can replace hydrogen by interchange of two atoms of metal for one atom of hydrogen. This tendency is manifested principally by mercury and copper; also, though to an inconsiderable extent, by silver and lead. These semiatomic metals are likewise monatomic; we have therefore to place mercury and copper, under the simple molecule, as semiatomic and monatomic; thus,— Hg', Hg'; Cu', Cu'.

Hg, Hg; Cu,,Cu.

459. The following metals have also two rates of exchange for hydrogen. We give them here as monatomic metals,

Pt', Pt'; Au', Au'; Cr', Cr′; Fe', Fe'; Mn', Mn'; U', U'; Pd', Pd'.

(2.) ALCOHOLIC HYDRIDES. ALCOHOLIC METALS.

1ST CLASS.

(General Formula of the Radicals of this class Cn H2n + 1.)

C2 H2

460. The most general formula of vinic alcohol is that which represents it on the water type; in this formula one atom of the hydrogen in water is replaced by the hydrocarbon C, H,, thus H O; similar formula will, therefore, be the most general for all the members in this group of alcoholic bodies. The composition of the hydrocarbon in each alcohol is peculiar to that one; it exists in no other alcohol; it differs from the one in the alcohol next lowest in the series by an increase of CH, and from the one next above it by a decrease of C H2; thus the formula for the methylic alcohol is CHO; the formula for ethylic alcohol, which is next above it in the C, H, series, is O; and the formula for the alcohol next

H

Н

above, vinic alcohol, is CHO; the general formula for

this group of alcohol radicals is Cn H2n + 1; Cn Hn signifying equal atoms of carbon and hydrogen.

461. When one atom of hydrogen in the molecule of hydrogen is replaced by an atom of one of these alcohol radicals, we get the hydrides of these radicals. Example. -Hydride of ethyl H, C, H,. When the other atom of hydrogen is replaced by a metal, we get a compound formed on the type of the molecule of hydrogen, HH, in which one atom of hydrogen is replaced by a metal, and the other atom by one of the alcohol radicals. Example. -Zinc-ethyle, Zn, C, H,. When the two atoms of hydrogen in the molecule of hydrogen are replaced by two

A series of analogous substances, whose composition varies by CH,, or a multiple of it, is called a series of homologous bodies. See Appendix A, page 278.

atoms of one of these alcohol radicals, we get the molecule or molecular atom of these radicals, or, as Gerhardt calls them, the alcohol metals. Example.-Ethyle-ethyle, CH, C, H. When the two atoms of hydrogen in the molecule of hydrogen are replaced by different alcohol radicals, we get mixed alcohol metals. Example.-Ethylamyl, CH, C, H. The hydrides are called, according to Gerhardt, the primary derivatives, and the alcohol metals the secondary derivatives of hydrogen.

462. We shall now give a list of all the hydrides of the alcohol radicals and all the alcohol metals of this class which have yet been obtained in an isolated state; we shall afterwards see that there are good reasons for believing that the list will be extended.

463. The student must commit to memory the names and symbols of these alcohol radicals; he will then not only be able to write out the formulæ of the alcohol metals and the hydrides, but he will also be prepared to write out all the other combinations of these radicals. These bodies have at present two sets of names; those describing their position in the series, as trityl, tetryl, have been given them by Gerhardt; the others were given at the time of their discovery.