Lactic acid is a trisubstituted derivative of marsh gas of the form

CH,

H-C-OH

со-он Lactic acid.

In this case the plane passing through CH,, C, and CO-OH will no longer be a plane of symmetry, for of the two symmetrical hydrogen atoms which existed in propionic acid one has been replaced by OH; the molecule will therefore be dissymmetrical. Lactic acid exercises a rotatory power.

This is also true for tartaric acid, which may equally be regarded as a trisubstituted derivative of marsh gas, assuming the general form

R

H-C-R'

R".

Tartaric acid being

CO-OH

H-C-OH

CH-OH

CO-OH,

we see that one of the carbon atoms is united to a hydrogen atom, and to three different groups, R" being

in this case |

CH.OH

CO.OH

From what has been said above,

the structure of the molecule is dissymmetrical.

The objection will, however, be made that the lactic acid of fermentation is inactive, as are also mary other Lodies presenting a similar structure. Le Bel sets this difficulty aside by observing that, in a bisubstituted derivative of marsh gas, the third substitution may take place on either side, to left or to right, and that the dissymmetrical compounds thus formed are in reality a mixture in equal proportions of dextro-rotatory and lævo-rotatory bodies, and are consequently inactive.

2

We may add that the above course of reasoning implies an hypothesis that, namely, of the fixity of hydrogen atoms and groups, relatively to each other, in the types CHR,R′ and CHRR'R". If this were not so, if the hydrogen atoms and the groups which replace them could be continually changing places in this system and alternately occupy all positions, the molecular dissymmetry, as defined above, could produce no effect, because in the mass of the molecules we should continually find dissymmetrically opposed compounds in equal proportions, and consequently optical neutrality. On the other hand, the grand fact of isomerism demonstrates the stability of the relative positions of atoms and groups for an immense number of combinations. It is well known that to determine, in a given combination, the migration of atoms or groups which produces isomerism, the intervention of a physical force or a chemical agent is necessary for example, a great elevation of temperature. How could the innumerable isomers of the innumerable derivatives of benzene occur, if the six hydrogen atoms of benzene were not all riveted to their carbon atom, executing in its vicinity, without ever

quitting it, as long as the compound exists, those motions which constitute a part of the total energy of the molecule ?

We shall confine ourselves to these brief observations, which will at least have shown the cause of the interest which attaches to the attempt of Le Bel and Van't Hoff.

Biot often insisted upon molecular dissymmetry as the cause of rotatory power, and upon the assistance which this remarkable physical property of certain bodies would one day offer to the study of their constitution. This was a prediction which was fulfilled when Pasteur showed the relation which exists between rotatory power and crystalline dissymmetry. But with liquid or dissolved substances it is no longer a question of crystals, but of molecules, and the considerations relative to molecular dissymmetry were, as it were, the offspring of pure chemistry. They have been deduced from the theory of atomicity, and are connected with modern ideas. upon the links existing between atoms in combinations.

CHAPTER IV.

HYPOTHESES UPON THE CONSTITUTION OF MATTER.

I.

We have now come to the end of this long exposition, and must conclude.

We have pointed out the origins and followed up the development of this celebrated theory of atoms, which, from the first ages of civilisation, has been present to the human mind, seeking both to penetrate into the inmost recesses of matter and to sound the infinite depths of space. And we have a sound reason for comparing the little world' in which atoms are rotating to the great world in which the stars revolve. In both all is motion. We must go back to the very origin of atomism to find this conception of atoms in motion. It is mental power (voûs) which gives them the impulse, according to Anaxagoras. According to Democrites of Abdera, they are in perpetual motion from their very nature; the force which animates them acts inevitably. They do not differ in their essence— we should now say, in their chemical qualities-but rather in their dimensions, for they have a sensible extension; they differ also in their form. If heavy, they fall into the depths of space; if lighter, they rise

in the air. Some have a smooth surface; others present asperities, points, and hooks. The motion which they possess naturally brings them into contact, without their attracting each other; sometimes it masses them together, sometimes separates them. It is in this manner that all things are formed or destroyed. Limited in extent and surface, they cannot be confounded with the medium in which they move. medium is a vacuum.

6

This

Thus we find, at the very origin of atomic ideas, this distinction between vacuum and plenum,' or vacuity and solidity,' which was maintained for ages, and which appears as one of the solutions which the human mind has been able to give of the constitution of matter. This is the hypothesis of the discontinuity of matter, now generally adopted, with the difference that the vacuum is replaced by a very rarefied, elastic, vibrating medium-ether.

Another hypothesis is that of continuous matter filling all space, with different degrees of density. Descartes inclined towards the latter hypothesis. Dimension being the essential property of bodies, there cannot be bodies without dimensions, which excludes the idea of indivisible particles-that is to say, of atoms. There is no dimension without body, consequently no

vacuum.

The same conception follows from the dynamic hypothesis professed by German philosophers at the commencement of this century. Whether, after Kant, matter exists by itself, or is endowed with two contrary forces, one attractive and the other repulsive, or