If we wish to distinguish between two such “allotropic modifications” apart from any theoretical views concerning the nature and constitution of matter, we can say that such “modifications” are different because equal weights of them contain, or are equivalent to, different quantities of energy,[129] since the change of one “form” to another takes place only with the evolution or absorption (as the case may be) of heat.[130] But, according to modern views regarding the nature of matter, this is the sole fundamental difference between two different elements--such are different because equal weights of them contain or are equivalent to different quantities of energy. The so-called “allotropic modifications of an element,” therefore, are just as much different elements as any other different elements, and the change from one “modification” to another is a true transmutation of the elements; the only distinction being that what are called “allotropic modifications of the same element” differ only slightly in respect of the energy they contain, and hence are comparatively easy to convert one into the other, whereas different elements (so called) differ very greatly from one another in this respect, whence it is to be concluded that the transmutation of one such element into another will only be attained by the utilisation of energy in a very highly concentrated form, such as is evolved simultaneously with the spontaneous decomposition of the radium emanation. [129] For a defence of the view that chemical substances may be regarded as energy-complexes, and that this view is equally as valid as the older notion of a chemical substance as an inertia-complex, _i.e._, as something made up entirely of different units or atoms each characterised by the possession of a definite and constant weight at a fixed point on the earth’s surface, see an article by the present writer, entitled “The Claims of Thermochemistry,” _Knowledge and Scientific News_, vol. vii. (New Series), pp. 227 _et seq._ (July, 1910). [130] In some cases the heat change accompanying the transformation of an element into an “allotropic modification” can be measured directly. More frequently, however, it is calculated as the difference between the quantities of heat obtained when the two “forms” are converted into one and the same compound. Conclusion.