hydrogen in the molecules of water, and, measured by the heat evolved, this force is nearly equaled by the force which unites the similar atoms of nitrogen to form a molecule of nitrogen gas; and the great violence of many modern explosives depends upon this circumstance.
It will now be seen that with our new philosophy the whole glamour which formerly bedazzled our idea of an elementary substance, and distinguished it widely from all other substances, disappears. The differences between substances depend upon the differences between their molecules, and as great molecular differences may arise from the union of similar as from the union of dissimilar atoms. The union of two atoms of hydrogen and one of oxygen gives a molecule of water, the union of two atoms of hydrogen and two of oxygen gives a molecule of peroxide of hydrogen; the union of two atoms of oxygen alone gives a molecule of oxygen gas, the union of three atoms of oxygen a molecule of ozone, and the difference between the last two substances is as great and of the same sort as the difference between the first two; and so it is with the so-called allotropic states of other elementary substances.
According to the modern philosophy of chemistry, the properties and relations of a substance depend fully as much upon the manner in which the atoms are grouped in the molecules of the substance as upon the nature of the atoms of which the molecules consist; and the differences between isomeric substances to which we have referred, depend wholly on what we call the molecular structure. The molecules, both of butyric acid and of acetic ether, consist of four atoms of carbon, eight atoms of hydrogen, and two of oxygen, and the chemist will show you just how these atoms are grouped in the molecule of each substance, and how the separate relations of these widely differing products depend on the structure he has assigned to their respective molecules. Indeed, the study of molecular structure—that is, of the mode of grouping of atoms in the molecules, especially in those of the compounds of carbon—has almost engrossed the attention of chemists for the past twenty-five years. An immense mass of facts and theories has been collected, and a symbolical method of representing the structure has been adopted, which, although highly conventional, must embody real truth, however dimly it may be now perceived; for the system has led to more, and more important, discoveries than any one of the dominant systems of science of the present day. The system has a great charm for students, and what is called the study of organic chemistry in our colleges is wholly a discussion of problems of this kind.
These systems of atoms that we call molecules have been frequently compared to the solar system, and cited as evidence that