In connexion with our discussion of electricity (Chapter XIII) we spoke of the special function of the metals as bearers of the 'mercurial' quality (in the alchemical sense of the term). As one of the characteristics which reveal this function we mentioned the peculiar capacity of metals to behave as 'solid fluids'. This exceptional place among the mineral substances of the earth, the metals owe to their close association with the extra-terrestrial astral forces of the world. In this field, too, modern spiritual investigation has recovered something which was known to people of old - that among the metals there are seven which have a distinctive character, for each stands in a special relation to one of the seven planets (that is, the planetary force-spheres) of our cosmic system. This is shown in the following table:

 Saturn Lead Jupiter Tin Mars Iron Sun Gold Venus Copper Mercury Quicksilver Moon Silver 

Among the discoveries of the last century in the realm of acoustics, there is one which especially helped to establish a purely kinematic conception of sound. Helmholtz showed that tones which to our ears seem to have a clear and definite pitch may be split up by a series of resonators into a number of different tones, each of them sounding at a different pitch. The lowest of these has the pitch which our ears attach to the entire tone. Thus in any ordinary tone there may be distinguished a 'fundamental' tone and a series of 'overtones'. Helmholtz further showed that the particular series of overtones into which a tone can be resolved is responsible for the colour of that tone as a whole. Naturally, this meant for the prevailing mode of thinking that the experience of the colour of a tone had to be interpreted as the effect of a kind of acoustical adding together of a number of single tone perceptions (very much as Newton had interpreted 'white' light as the outcome of an optical adding together of a certain number of single colour perceptions).

Our discussion has reached a point where we are able to answer a question which first arose in the course of our study of the four ethers, and which arises here anew.

When Kepler, against the hopes of his forerunner and friend, Tycho Brahe, accepted the heliocentric standpoint and made it the basis of his observations, he did so out of his understanding of what was the truth for his own time. Kepler's ideal was to seek after knowledge through pure observation. In this respect Goethe took him as his model. Kepler's discoveries were a proof that man's searching mind is given insight into great truths at any stage of its development, provided it keeps to the virtue of practising pure observation.

In this way the equation -
r₁³ / r₂³ = t₁² / t₂²
assumes the form-
r₁³ / r₂³ = (t₁ / t₂) / (t₂ / t₁)
The right-hand side of the equation is now constituted by the double ratio of the linear values of the periods of two planets, and this is something with which we can connect a quite concrete idea.

By means of these observations we realize that the third type of force, in so far as it is active in man, has the capacity, by co-ordinating the physical and etheric parts of the organism in one way or another, to promote happenings either of a more corporeal or a more psychical nature - namely, motion at one pole, sensation at the other, and feeling in the middle between them.¹ Remembering Goethe's formula, 'colours are deeds and sufferings of light', we realize how deeply true the concepts were to which he was led by his way of developing observation and thought.

The implications for us of these remarks by an eminent physicist can be expressed as follows:

As compared with these seven, the other metals are products of combinations of various planetary forces. A comparison of the role of Saturn as the outermost planet of our cosmic system with the role played by its metal, lead, as a final product of radioactive disintegration, leads one to conceive of the radioactive sphere of the earth as being related especially to the planets outside the orbit of Saturn, namely, Uranus, Neptune, Pluto.

The picture becomes different if we apply to the aural experience Goethe's theorem that, in so far as we are deluded, it is not by our senses but by our own reasoning. For we then realize that sounds never occur of themselves without some tone-colour, whilst physically 'pure' tones - those that represent simple harmonic motions - exist only as an artificial laboratory product. The colour of a tone, therefore, is an integral part of it, and must not be conceived of as an additional attribute resulting from a summing up of a number of colourless tone experiences.

In studying the chemical or sound ether we were faced with the fact that part of the etheric realm, although in itself accessible to the spiritual part of the sense of sight, offers supersensible experience comparable to the perception of sound. Conversely, we are now met by the fact that it is spiritual hearing which gives access to the immediate perception of a realm of forces which is not only the source of acoustic phenomena, but the origin of all that manifests in nature in the form of sulphurous, saline and mercurial events, such as the world of colours, electricity, magnetism, the manifold rhythmic occurrences on the earth (both taken as a whole .and in single organisms), etc. - all of which are taken hold of by quite other senses than that of hearing.