MOLECULE, CHEMICAL. The chemical molecule is us ually defined as the smallest part that exists free in the gaseous form of a chemical substance. A very large number of chemical substances, however, have never been obtained in the gaseous form, and, in fact, many substances are incapable of existence ex cept in the solid form. The compound chemical substance, alum, for example, decomposes on melting and exhibits no trace of vol atility; it can be subdivided mechanically almost an infinite number of times without any change in chemical properties ap pearing, though it can be ultimately resolved by chemical processes into the five elementary substances, potassium, aluminium, sul phur, oxygen and hydrogen. Obviously there must be some stage in the subdivision of alum at which further subdivision will de stroy the individual substance and reveal the presence of the elementary constituents : this ultimate stage is the stage of the molecule. A molecule thus represents the limit of subdivision of a substance at which the chemical properties are wholly retained.
Chemical science is not directly concerned with the absolute sizes of either molecules or atoms, all molecular and atomic weights being multiples of an arbitrary unit. The element selected as the standard was originally hydrogen, as the lightest known gas. For purposes of convenience (accessibility of measurement, etc.), the modern standard is the gas, oxygen, which is approxi mately 16 times as heavy as hydrogen. Taking oxygen as 16, the atomic weight of hydrogen is i.008, which for most practical purposes is taken as unity. Though chemistry is concerned only with these relative weights, it is possible on chemical considera tions to obtain some very approximate idea of the absolute weights of molecules and atoms. If one grain by weight of the colouring matter, fluorescein, is dissolved in 354 gallons of water, the yellow colour is just perceptible in a single drop of the solution. This drop contains i/5oo,000,000th part of the original fluorescein. Though this dilution is at the visual limit of fluores cein detection, it is still very far from the molecular stage. If the single drop, however, is diluted with water a further four million million times, a drop of the resulting solution would con tain a molecule of fluorescein, and it is known by chemical analy sis that the fluorescein molecule is 332 times as heavy as a hydrogen atom.
In the early days of chemical science the distinction between atoms and molecules was not appreciated. John Dalton, the founder of the atomic theory, in i8o8 regarded the atom as the smallest ultimate weight of both elements and compounds. He
thus referred, for example, to the ultimate particle of the com pound of nitrogen and hydrogen as an atom of ammonia. The term atom is no longer used in connection with compound par ticles, and is reserved solely for the ultimate particles of elements.
Dalton assumed that the elementary gases were composed of individual atoms, and that chemical combination occurred simply atom to atom. In the same year L. Gay Lussac discovered the "law of simple multiple proportions by volume," which, in modern terms, states that the gaseous volumes of substances taking part in a chemical change and the total change of volume, if any, are small integral multiples of a common factor of these volumes. In a very great number of cases it was known that the volumes of gases taking part in chemical changes were equal, and in 181I Amadeo Avogadro put forward the hypothesis that equal volumes of gases under the same conditions contain the same numbers of particles. Avogadro used the term molecule in the sense in which Dalton had used the term atom, and distinguished the gas particle by the term integral molecule. The simple term molecule is now reserved solely for gas particles and ultimate particles of com pounds. By means of this hypothesis as to gas particles, Avoga dro was enabled to discern that the gas particles of nearly all the elements were made up of more than one ultimate particle or atom, the majority of the elements possessing diatomic particles or molecules. The molecules of hydrogen and chlorine, for example, were deduced to be double, on the ground that the gas eous hydrogen chloride they formed had double the volume of the hydrogen or chlorine used in the combination. Similarly, oxygen had a diatomic molecule, because the volume of water vapour was double the volume of oxygen used or obtainable from it, and nitrogen likewise, because ammonia gas was double the volume of nitrogen obtainable from it. Avogadro's hypothesis and his deduc tions therefrom were disregarded, and confusion about atomic and molecular weights prevailed in chemistry for nearly half a cen tury. But the hypothesis was revived in 1858 by another Italian, Stanislao Cannizzaro, who showed that all known physical and chemical facts confirmed its validity, and that the inferential dis tinction made possible between atoms and molecules reconciled all the contradictory experimental results accumulated since the beginning of chemistry as a science.