This behaviour of the element appears to show that it also is a compound, but that it is stable under ordinary conditions, and is only dissociated at a high temperature.

Other proofs of this hypothesis are derived from a comparison of the spectra of the elements as observed in our laboratories with their spectra in the sun.

A comparison of the two hypotheses shows us that as on the old hypothesis each element represents a species and is unvariable, its spectrum ought to be always the same in our laboratories and in the sun : and the same in sun-spots as in prominences, and the same at all periods of the sun's activity.

supposed to depend on the difference in the free path of the molecule. But according to the new view, the difference in the complexity of the molecule itself is sufficient to explain the phenomena.

Under the new hypothesis the spectra of metals in our laboratories and in the sun should not resemble each other; they should be different in sun-spots and in prominences because the spot is cooler than the prominence; and they should vary at the time of the sun's activity because the sun is hotter at the maximum of the sun-spot period, and therefore there should be a greater amount of dissociation amongst the elements at that period

As a matter of fact we find that the spectra in our laboratories and in the sun do not resemble each other (Fig. 2); that those of the same element in the sun-spot and prominences are as dissimilar as of any two elements; and that the spectra of the elements in the sun do vary with the maximum of the sun-spot period.

Fig. 2.   Diagram of the spectrum of lithium under various conditions of temperature

Fig. 2. - Diagram of the spectrum of lithium under various conditions of temperature (After Lockyer, Roy. Soc. Proc. Dec. 12, 1878.)

On the old hypothesis the spectra of prominences should also consist of lines familiar to us in our laboratories, because solar and terrestrial elements are the same, while, according to the new hypothesis, the spectra of prominences should be unfamiliar, because the prominences represent outpourings from a body hot enough to prevent the atoms of which our elements are composed from coming together.

As a matter of fact, the lines in the prominences, with the exception of those of hydrogen, magnesium, calcium, and sodium, are either of unknown origin, or are feeble lines in the spectra of known elements. Spectroscopic observation, therefore, leads to the belief that the so-called elements are really compounds, the component parts of which are kept apart by high temperatures in the sun and stars, but unite when the temperature decreases.

By the powerful vibrations imparted to them by the electric spark, they may be dissociated in the laboratory; but, as no means has yet been devised of separating the components, they again unite to form the original body, just as hydrogen and oxygen, into which steam is dissociated by passing it through a strongly heated tube, almost instantly combine again to form water unless they are separated by means of the more rapid diffusion of hydrogen through a porous tube.

The difficulty in accepting this evidence lies in the fact that we have hitherto been unable to isolate the substances into which the elements are supposed to be dissociated: as these after their dissociation at once recombine and again form the original substance.

One proof, however, that the supposed components of the element calcium may remain permanently separated, is afforded by the fact that in the spectra of two stars, Sirius (Fig. 3) and a Lyrae, which are very bright, and probably very hot, only one of the ultra-violet lines of calcium is represented.

Fig. 3.   Diagram of the spectrum of calcium under various conditions of temperature

Fig. 3. - Diagram of the spectrum of calcium under various conditions of temperature. In the spectrum of Sirius the line k is absent, while it is very strongly marked in the solar spectrum.

But we have also other evidence of the compound nature of the elements, which, although it was not sufficient of itself to force us to abandon our old ideas of their simple nature, is yet strongly corroborative of the spectroscopic evidence. Thus we find that oxygen is broken up by electricity, and that the atoms of which its molecules are composed, rearrange themselves so as to form what is to all intents and purposes a new element, ozone, having a much closer resemblance to chlorine than to oxygen in its activity, although its compounds with metals appear to be identical with those of oxygen.

a

a

b

b

Fig. 4. - Diagram to illustrate the formation of ozone by electricity. a represents oxygen, through which a spark is passing; b after it has passed. The double rings are intended to represent molecules of oxygen, each containing two atoms. As the electric spark passes through the oxygen it breaks up the first molecule, carrying one atom on to join the second molecule of oxygen, and form one of ozone. The atom which is left joins another molecule of oxygen, and also forms ozone. (After Lockyer.)

At a high temperature its atoms are again dissociated, and recombine to form ordinary oxygen. When it combines with other substances, the heat of combination appears to be sufficient to dissociate the atoms of ozone (O3), so that in the compound we meet with simple oxygen, O.

When sulphur is simply melted and cooled, it solidifies as a yellow brittle substance, but if it is heated to 200° it becomes brownish and thick, and if it be suddenly cooled, by throwing it into water, it solidifies as a transparent reddish plastic and elastic substance. The ordinary brittle and yellow, and the reddish plastic sulphur, appear to be quite different substances. But if the plastic sulphur be left for some hours, it becomes reconverted into ordinary sulphur; and if either ordinary or plastic sulphur be volatilised, the vapour condenses in the form of ordinary sulphur; but if the vapour is quickly cooled, the sulphur, while retaining its ordinary appearance, may yet undergo a certain change evidenced by its becoming insoluble in bisulphide of carbon. On the new hypothesis we explain these phenomena by supposing that the different forms of sulphur are different compounds, or perhaps we should rather say different aggregates, for their components may not differ in kind like those of calcium, but only in number like those of oxygen or ozone.

Indeed we are almost driven to such a conclusion by the behaviour of sulphur in regard to its vapour density, for only at very high temperatures does the specific gravity of the vapour follow the general rule, and at lower temperatures.it is three times as great as it ought to be, indicating that at these lower temperatures the molecule of sulphur contains six atoms instead of two.

Phosphorus also affords us an example of an element which occurs in two forms, so different that we should call them distinct bodies, were it not that we find that one can be transformed into the other.

The two forms, red and yellow phosphorus, differ from each other, not only in their colour, but in their density, specific heat, readiness of combustion, and heat of combustion. They differ also in the fact that yellow phosphorus is exceedingly poisonous, whereas the red phosphorus is not poisonous. They are in many respects, then, different bodies, but we have hitherto been content to call them allotropic forms of the same element.

In combination we find that phosphorus is sometimes pentad and sometimes triad; that its compounds with oxygen are sometimes poisonous, at other times not. Thus orthophosphoric acid, H3PO4, is not poisonous; pyrophosphoric acid, H4P2O7, and metaphosphoric acid, HPO3, are both poisonous.

The most striking example, however,is carbon, which we not only find in three forms, differing enormously from each other, as diamond, charcoal and graphite, but which we find in various compounds playing the most varied parts. This we at present explain by saying that carbon unites with itself in the formation of the various radicals; and thus comes to form what are practically new elements.

Another example is afforded us by ammonia, the salts of which are just as well characterised as those of potash or soda. The amalgam which it forms with mercury possibly indicates that we have in it also a real metal, ammonium, corresponding to sodium or potassium, though this is uncertain!

The three metals, sodium, potassium, and ammonium (if it exist), agree in the readiness with which they are oxidised, so that it is difficult to preserve the pure metal, although the oxide is stable. They differ, however, in the oxides of potassium and sodium being solid, and that of ammonium gaseous. Ammonium has not been isolated, and it is put down in the text-books as a hypothetical substance, but ammonium salts are tangible enough, and the question which we have to keep before us is, whether potassium, sodium, and all the other so-called elements, are not in reality compounds like ammonium.

Some people still regard species as immutable, and look upon Darwin's hypothesis of evolution as unproven.

The evidence in favour of the evolution of elements from one simple form of matter, is as yet, perhaps, much less strong than that in support of the evolution of species; but the hypothesis has this advantage, that it explains certain phenomena which have hitherto been very perplexing.

It may be at least convenient in discussing the physiological action of drugs to bear this hypothesis in mind, and to remember that what we have hitherto been accustomed to call elements may be really constituted like the so-called organic radicals, with this difference, that we can split up organic radicals with tolerable facility, while we cannot do this - at least to any great extent - with elements.

It also shows us that we must as pharmacologists pay attention to molecular as well as to empirical composition, and take into consideration crystalline form and physical aggregation in all observations regarding the relations between elements or compounds and living organisms. It is not sufficient, for example, to speak of the action of phosphorus on the organism as if this were invariable, for it varies with the molecular composition of the body in the red or yellow form, and isomeric organic substances may be utterly different in action.