This clearly points to some sort of community of origin, and thus favours the idea that the elements are in reality composite structures. But the great difficulty felt by those who have favoured this idea has been the apparent impossibility of decomposing such family groups. Thus fluorine, chlorine, bromine, and iodine, while they appear to be related to one another in some peculiar manner, have yet apparently resisted all attempts at decomposition, and there are other similar instances which might easily be named. 158. It has, however, at the same time, come to be recognised, that heat of high temperature is a very powerful decomposing agent, and that its office is by no means limited to causing the separation from one another of the molecules of a substance, as, for instance, when it separates the molecules of water-substance or H_{2}O from one another, as in forming water from ice, or steam from water. It is now understood that high-temperature heat has also the power of separating the atomic constituents of a single molecule from each other, so that at an extremely high temperature not only would water be driven into steam, but steam driven into oxygen and hydrogen. We are already familiar with many instances of this power possessed by high-temperature heat; thus we see carbonate of lime decomposed by the heat of the kiln into lime and carbonic-acid gas. We see also that at the high temperatures which accompany the electric spark almost all compounds are momentarily decomposed, if we may judge by the spectrum of the light which is given out. Carrying on this line of thought, we are led to imagine that, could we obtain higher temperatures than those now at our disposal, we might decompose some of those substances which at present seem to be elements.