As has been said, in all this symmetrical structure of the atoms of the elements, Bohr has in many cases had to rely upon general considerations of the information that observation gives about the properties of the individual elements. It must, however, not be forgotten that the backbone of the theory is and remains the general laws of the quantum theory, applied to the nucleus atom in the same way as they originally were applied to the hydrogen atom, leading thereby to the interpretation of the hydrogen spectrum. We have, further, a most striking evidence as to the correctness of Bohr’s ideas in the fact that not only do the pictures of the atoms which he has drawn agree with the known chemical facts about the elements, but they are also able to explain in the most satisfactory manner possible the most essential features of the characteristic X-ray spectra of the different elements, a field we shall not enter upon here. In all that has been said above we have been considering the Bohr theory simply as a means of gaining a deeper understanding of the laws which determine activities in the atomic world. Perhaps we shall now be asked if we can “utilize” the theory, or, in other words, if it can be put to practical use. To this natural and not unwarranted question we may first give the very general answer, that progress in our knowledge of the laws of nature always contributes sooner or later, directly or indirectly, to increase our mastery over nature. But the connection between science and practical application may be more or less conspicuous, the path from science to practical application more or less smooth. It must be admitted that the Bohr theory, in its present state of development, hardly leads to results of direct practical application. But since it shows the way to a more thorough understanding of the details in a great number of physical and chemical processes, where the peculiar properties of the different elements play parts of decided importance, then in reality it offers a wealth of possibilities for making predictions about the course of the processes—predictions which undoubtedly in the course of time will be of practical use in many ways. In this connection the discovery of the element hafnium, discussed on p. 204, may be mentioned. It must be for the future to show what the Bohr theory can do for technical practice. Below is given an explanation of the different symbols which occur at various places in the book; also the values of important physical constants. 1 m. = 1 metre; 1 cm. = 1 centimetre = 0·394 inches. 1 μ = 1 micron = 1/1000 of a millimetre = 0·0001 cm. = 10⁻⁴ cm. 1 μμ = 1/1,000,000 of a millimetre = 10⁻⁷ cm. 1 cm.³ = 1 cubic centimetre. 1 g. = 1 gram; 1 kg. = 1 kilogram = 2·2 pounds. 1 kgm. = 1 kilogrammetre (the work or the energy required to lift 1 kg. 1 m.). 1 erg = 1·02 × 10⁻⁸ kgm. = 7·48 × 10⁻⁸ foot-pounds. λ represents wave-length. ν represents frequency (number of oscillations in 1 second).