elements helium, carbon, nitrogen, oxygen, fluorine, phosphorus, sulphur, arsenic, iodine and sodium are found to be simple bodies with whole-number atomic weights. On the other hand, boron, neon, silicon, chlorine, bromine, krypton, xenon, mercury, lithium, potassium and rubidium are found to be mixtures. What is specially of interest is that the indicated atomic mass of each of the constituents is a whole number. Thus chlorine, whose atomic weight is 35·46, is found to be a mixture of two chemically-identical elements whose atomic weights are 35 and 37. Some of the elements, _e.g._, xenon, are mixtures of more than two isotopes. [93] Hon. R. J. STRUTT: “On the Tendency of the Atomic Weights to approximate to Whole Numbers,” _Philosophical Magazine_, [6], vol. i. (1901), pp. 311 _et seq._ [94] F. W. ASTON: “Mass-spectra and Atomic Weights,” _Journal of the Chemical Society_, vol. cix. (1921), pp. 677 _et seq._ It is highly probable that what is true of the elements investigated by Dr. Aston is true of the remainder. It appears, therefore, that the irregularities presented by the atomic weights of the ordinary elements, which have so much puzzled men of science in the past, are due to the fact that these elements are, in many cases, mixtures. As concerns hydrogen, it is only reasonable to suppose that the close packing of electrically charged particles should give rise to a slight decrease in their total mass, so that the atomic weights of other elements referred to H = 1 should be slightly less than whole numbers, or, what is the same thing, that the atomic weight of hydrogen referred to O = 16 should be slightly more than unity. The “Periodic Law.”