so-called organic forces. And the difference of conditions governing the formation of our inorganic lead ferns from those under which the organic vegetable fern is developed are not so entirely different and remote as you might at first think.

In the formation of the metallic fern we have the material suspended in the transparent solution of the acetate of lead. The material from which our vegetable fern is built up is also suspended in the atmosphere as one of the forms resulting from the union of carbon and oxygen. The electric current liberates, or furnishes the initiative for liberating, the acetic acid, leaving the atoms of lead free to obey the impulse of their affinities, and they unite and grow into beautiful forms. In the leaves of the vegetable fern the sunlight, a force akin to electricity, liberates the oxygen from the carbonic acid and aqueous vapor, and leaves the carbon and hydrogen free to unite in the building up of the plant. In both instances the molecular attractions find expression in similar forms, and the substances are acted upon by similar forces in a similar manner.

By a very ingenious calculation, based upon the decomposition of light in the film of a soap-bubble, Sir Wm. Thompson estimates the diameter of a molecule of water to be not far from -5iyxr,iFiTir,Tnnr °^ an inch. Not only is the molecule a definite mass which may be measured, but it can also be weighed. The unit of molecular weight is the microcrith, which is the weight of the hydrogen half-molecule. A single molecule of oxygen, therefore, weighs 32 microcriths; one of nitrogen, 28; and one of chlorine, 71. Now, if you have well fixed in your minds the truth that a molecule is a mass of matter of definite size and weight, composed of atoms possessing one or more poles of attraction, by means of which they unite, let us study a little the structure of the molecules of a few substances. You have here (" The New Chemistry," Cooke, p. 242) a table showing the poles of attraction of some of the elements. These marks indicate the poles, combining power, or quantivalenee of the atoms. The number is not invariably fixed in all of them; for instance, we see that A is placed in the column of those elements having three poles, I but in some combinations N is known to have five poles.

? This diagram shows the construction of a molecule of amn~~N monia gas. Each of the three poles of the N has united with

H the single pole of an atom of H.

^ The second diagram shows N with five poles, four of which

if/N—ci have united with H, and the remaining pole with CI to form

H ammonic chloride.

But these exceptions are circumscribed by limits which are easily defined. In all compounds, it is supposed that the atom having

the highest or strongest quantivalenee, or largest number of poles, takes a position in the center. In complex molecules there are sometimes several centers, around which the other atoms are grouped, much as some of the irregular bones are developed from several centers; in fact, as we shall see a little further on, certain atoms, or combinations of atoms, do really serve as a kind of skeleton for the building up of the molecule.

Ca/°\s/° In this diagram it will be seen that the atom of S, which 0 ° is sexivalent, or has six poles,* occupies the center, and holds the atoms together, forming a molecule of calcic sulphate.

° o Here, in the next diagram, we have a very

s complex molecule. You will observe that

° ° ° o two atoms of aluminum with their four

K—o—s—o—Ai—Ai—o-s—o—K poles each form the center of the group.

ooo o There are also four secondary centers

s formed by the four aton?s of sulphur, to the

o o poles of which are united atoms of oxy

gen, and to the oxygen two atoms of potassium, the whole forming a molecule of potassic-aluminic sulphate,f or alum. JSTot only are atoms endowed with poles of attraction, but the poles, as I have already remarked, are electro-positive and electro-negative, as shown by that interchange or substitution of atoms, known in chemistry as metathesis.

The radicals of the alkalies have been found to be electro-positive, while the radicals of the acids are electro-negative. Again, our magnet furnishes a beautiful illustration of the manner in which these electro-negative and electro-positive atoms act. If a soft iron bar is brought in contact with a powerful magnetic pole, we find that the two ends of the bar become strongly polar, the end furthest from the pole of the magnet having the same electrical condition as the active pole of the magnet. Now, if we place a lump of nickel at the free end of the iron bar, it becomes magnetized and adheres to the bar. Let us suppose that this lump of nickel is as large as the iron is capable of magnetizing or holding. If we now bring near the pole of the bar which is holding the nickel a piece of soft iron, the bar drops the nickel and takes the iron. This simple experiment forms a striking illustration of the manner in which atoms are often substituted for other atoms in the construction of molecules.

Suppose we have an atom of oxygen, O, with its two poles. An atom of potassium, K, which is a strong, positive radical, is brought in contact with one pole of the oxygen atom. The opposite pole of this oxygen atom is then rendered strongly positive, and attracts the negative atom of H, forming a molecule of potassic hydrate. But the H being a somewhat indifferent element, is not held in very secure bonds. If a radical like N02, the radical of nitric acid, which is capable of receiving a greater degree of polarity, is brought in contact with this molecule, it drops the H and takes the radical, just as our magnet gave up the nickel for the iron. Passing over much that is interesting in this connection, let us turn to the extensive group of carbon compounds, which constitute what is known as organic chemistry, and which is more intimately connected with our subject.

The term nutrition, as applied to the formation and growth of

vegetables, may be said to be synonymous with the formation of

these carbon compounds. We have seen that the carbon atom has

four poles of attraction, which may be variously represented, as

I
—c—

I V " _c c Hnown DJ these diagrams. Suppose we satisfy or /\> n> 'IIB close the four bonds of a carbon atom thus, ** with four univalent hydrogen atoms. We have a molecule of H—c—H the gas commonly known as marsh-gas, or fire-damp, the H chemical name of which is methyl hydride, or methane. If, now, we substitute an atom of the univalent element CI for one atom of the H, we have:

H

H—c—ci

i

H

C H3 CI, a molecule of methyl chloride, or mono-chlor-methane. If we take away three of the hydrogen atoms and supply their places with three atoms of chlorine, we have:

ci
H—c—ci

<!,

C H Cl3, a molecule of trichlor-methane, or chloroform. There is a large group of these compounds founded upon this single atom of C, with its four poles, and known as the methane group. We also find that the carbon atoms unite with each other to form what are called radicals having from one to twenty or more poles of attraction, and each of these radicals is the skeleton, center, or nucleus of a large group of compounds. I have already given you illustrations of the methane, or Gv group. The C2 group is the ratlical of the ethane compounds, which are hydro-carbons, shown __c—i_ ky to*s diagram. You will observe that two of the bonds I I unite with each other, leaving six exposed. If we satisfy these six bonds with six atoms of hydrogen, we have ethane, or dimethyl. If we unite the carbon atoms by two of their bonds thus, (j=i anc* satisfy those four exposed poles with four hydrogen atomSj

1 I we have a molecule:

H H

I 1 c=c

I I

H H

C2 H4, of the colorless, poisonous ethylene, or olefiant gas. If we

interpose an atom of oxygen, which has two poles, between the two

l_ L atoms of carbon, thus, and then give the six poles of the

I I radical each an atom of hydrogen, we shall have a molecule,

H H

H—C—0—C— H

k A

C2 H6 O, of ethyl alcohol, common alcohol, or spirit of wine.

If, now, we take a molecule of ethane, or dimethyl, and remove one of the hydrogen atoms and give the pole an oxygen atom, and to the exposed pole of the oxygen atom, we give another 02 group, with five hydrogen atoms at its exposed poles, or, in other words, if we simply take two molecules of ethane, and removing one hydrogen atom from each, unite the two by an atom of oxygen, we shall have a molecule of

H H H H
H—C—C—0—C— 0—H

C4 H10 O sulphuric ether, or ethyl oxide. If we remove all of the hydrogen atoms except one from a C2 group, and give three of the poles three atoms of chlorine, and the remaining two each a hydroxy 1 group thus:

ci OH

CI—i—C—OH

j,i

C2 H CJ3 0~f-H2 O, we have a molecule of chloral hydrate. On this

same C radical we may build

H

H—c—o—c=o

k k

0, H4 02, a molecule of acetic acid, or one of

H—o—c—c—o—H

ii ll 0 o

02 H2 04, oxalic acid.

We have now built on this simple group, or radical C2, molecules of ethane, defiant gas, alcohol, sulphuric ether, chloral hydrate, acetic and oxalic acids. I have presented quite a number of radically different substances from this group, to show how wonderfully simple and orderly is this process of building molecules when we have obtained the key of interpretation. This key is the carbon radical and the law of the substitution of atoms by the action of a force akin to magnetism or electricity. The type of the hydro-carbon group, which has three atoms of carbon, thus, for its radi- ^J^Lical, or skeleton, is propane, shown in this diagram, III

H H H

H-UO-H

Hi

formed by joining eight univalent carbon atoms to the eight exposed poles of the carbon group. This is a large group, but I shall mention only a few of the more familiar, or well-known numbers. If we remove three of the hydrogen atoms from this group, and substitute for them one atom of oxygen and a hydroxyl group thus:

H H

H—c_ c—c—o—H

I I ii

H H 0

we shall have a molecule of propionic acid, C3 H6 02, a substance found in sweat, the fluids of the stomach, and the blossoms of milfoil. If we remove three hydrogen atoms from a molecule of propane and substitute for them three hydroxyl groups thus, it gives us a molecule of glycerine, C3 Hg 03.

H
H 0 H

H—0—0—C—C—0—H

iil

In the C4 group, or butane compounds, the typal molecule of which is represented in this diagram,

H H H H

H—0—0—G-C—H

un

{Gi If10, butane), we find the well-known butyric acid, too well known when we find it in rancid butter,

H H H

H=C—C—C—C—0—H

uu

(C4 H8 02), in which three of the hydrogen atoms are replaced by an atom of oxygen and a hydroxyl group; also, malic acid, found in apples, gooseberries, and other sour fruits. In the molecule of this substance the relation of the four carbon atoms which form the radical is changed somewhat, as shown in this diagram: