IN the year 1803, just a century ago, John Dalton delivered a series of scientific lectures in the Royal Institution during the course of which he doubtless laid before his audience a theory which he had recently devised for the purpose of connecting together the vast number of isolated chemical facts known at the commencement of the nineteenth century. This theory, of which the centenary is being celebrated during the present month by the Manchester Literary and Philosophical Society, is known as the atomic theory, and was destined to form the foundation upon which the whole superstructure of modern chemistry has been built. For our present purpose Dalton's theory may be briefly stated in the form of the following two principles:—(1) Every element is made up of homogeneous atoms of which the mass is constant; (2) chemical compounds are formed by the union of atoms of the various elements in simple numerical proportions. In accordance with Dalton's hypothesis, chemical substances may be mentally pictured by imagining the atoms as small spheres which have the power of aggregating themselves together under suitable conditions to form complexes or “molecules ”; thus, taking two similar spheres representing hydrogen atoms, in conjunction with a sphere of a different kind, representative of an atom of oxygen, a chemical representation can be given of the compound water, the molecule of which is composed of two atoms of hydrogen and one of oxygen. The original atomic theory offers no explanation of the observed fact that the atoms combine together in different proportions; this deficiency was remedied by the doctrine of valency enunciated by the late Sir Edward Frankland in 1852. Frankland supposed that the atoms of certain elements, such as hydrogen and chlorine, are unable to combine with more than one atom of any other element; these elements are termed mono-valent. Other atoms, such as those of barium and zinc, can become directly attached to at most two other atoms; these are the divalent elements. Tri-, tetra-, penta-, hexa-, hepta- and octa-valent elements can be similarly distinguished, the valency of hydrogen being taken as unity, in order to measure and define the saturation-capacity or the atom-fixing power of the atoms of the other elements. It will be clear that for rough diagrammatic purposes we may provide the spheres representing the atoms with as many wooden pegs as the element itself exhibits units of valency; compound molecules can then be represented by fitting the atoms together by means of the pegs representing the number of valency-units possessed by the various constituent atoms. By so doing a great advance is made upon the atomic theory of Dalton's time, and a mental picture is obtained of the way in which the atoms are connected together within the molecule itself.