§ =83.= There are also certain other facts which appear to demand such a modification of Dalton’s Atomic Theory as is found in the Electronic Theory. One of the characteristics of the chemical elements is that each one gives a spectrum peculiar to itself. The spectrum of an element must, therefore, be due to its atoms, which in some way are able, at a sufficiently high temperature, to act upon the ether so as to produce vibrations of definite and characteristic wave-length. Now, in many cases the number of lines of definite wave-length observed in such a spectrum is considerable, for example, hundreds of different lines have been observed in the arc-spectrum of iron. But it is incredible that an atom, if it were a simple unit, would give rise to such a number of different and definite vibrations, and the only reasonable conclusion is that the atoms must be complex in structure. We may here mention that spectroscopic examination of various heavenly bodies leads to the conclusion that there is some process of evolution at work building up complex elements from simpler ones, since the hottest nebulæ appear to consist of but a few simple elements, whilst cooler bodies exhibit a greater complexity. Views of Wald and Ostwald. § =84.= Such modifications of the atomic theory as those we have briefly discussed above, although profoundly modifying, and, indeed, controverting the philosophical significance of Dalton’s theory as originally formulated, leave its chemical significance practically unchanged. The atoms can be regarded no longer as the eternal, indissoluble gods of Nature that they were once supposed to be; thus, Materialism is deprived of what was thought to be its scientific basis.[98] But the science of Chemistry is unaffected thereby; the atoms are not the ultimate units out of which material things are built, but the atoms cannot be decomposed by purely chemical means; the “elements” are not truly elemental, but _they are chemical elements_. However, the atomic theory has been subjected to a far more searching criticism. Wald argues that substances obey the law of definite proportions because of the way in which they are prepared; chemists refuse, he says, to admit any substance as a definite chemical compound unless it does obey this law. Wald’s opinions have been supported by Professor Ostwald, who has attempted to deduce the other stoichiometric laws on these grounds without assuming any atomic hypothesis[99]; but these new ideas do not appear to have gained the approval of chemists in general. It is not to be supposed that chemists will give up without a struggle a mental tool of such great utility as Dalton’s theory, in spite of its defects, has proved itself to be. There does seem, however, to be logic in the arguments of Wald and Ostwald, but the trend of recent scientific theory and research does not appear to be in the direction of Wald’s views. Certainly, however, it appears that, on the one hand, the atomic theory