In almost every part of physical science there is some substance of powers pre-eminent for the special purpose to which it is put. Rock-salt is invaluable for its extreme diathermancy or transparency to the least refrangible rays of the spectrum. Quartz is equally valuable for its transparency, as regards the ultra-violet or most refrangible rays. Diamond is the most highly refracting substance which is at the same time transparent; were it more abundant and easily worked it would be of great optical importance. Cinnabar is distinguished by possessing a power of rotating the plane of polarisation of light, from 15 to 17 times as much as quartz. In electric experiments copper is employed for its high conducting powers and exceedingly low magnetic properties; iron is of course indispensable for its enormous magnetic powers; while bismuth holds a like place as regards its diamagnetic powers, and was of much importance in Tyndall’s decisive researches upon the polar character of the diamagnetic force.9 In regard to magne-crystallic action the mineral cyanite is highly remarkable, being so powerfully affected by the earth’s magnetism, that, when delicately suspended, it assumes a constant position with regard to the magnetic meridian, and may almost be used like the compass needle. Sodium is distinguished by its unique light-giving powers, which are so extraordinary that probably one half of the whole number of stars in the heavens have a yellow tinge in consequence.
It is remarkable that water, though the most common of all fluids, is distinguished in almost every respect by extreme qualities. Of all known substances water has the highest specific heat, being thus peculiarly fitted for the purpose of warming and cooling, to which it is often put. It rises by capillary attraction to a height more than twice that of any other liquid. In the state of ice it is nearly twice as dilatable by heat as any other known solid substance.10 In proportion to its density it has a far higher surface tension than any other substance, being surpassed in absolute tension only by mercury; and it would not be difficult to extend considerably the list of its remarkable and useful properties.
Under extreme instances we may include cases of remarkably low powers or qualities. Such cases seem to correspond to what Bacon calls Clandestine Instances, which exhibit a given nature in the least intensity, and as it were in a rudimentary state.11 They may often be important, he thinks, as allowing the detection of the cause of the property by difference. I may add that in some cases they may be of use in experiments. Thus hydrogen is the least dense of all known substances, and has the least atomic weight. Liquefied nitrous oxide has the lowest refractive index of all known fluids.12 The compounds of strontium have the lowest dispersive power. It is obvious that a property of very low degree may prove as curious and valuable a phenomenon as a property of very high degree.
The Detection of Continuity.
We should bear in mind that phenomena which are in reality of a closely similar or even identical nature, may present to the senses very different appearances. Without a careful analysis of the changes which take place, we may often be in danger of widely separating facts and processes, which are actually instances of the same law. Extreme difference of degree or magnitude is a frequent cause of error. It is truly difficult at the first moment to recognise any similarity between the gradual rusting of a piece of iron, and the rapid combustion of a heap of straw. Yet Lavoisier’s chemical theory was founded upon the similarity of the oxydising process in one case and the other. We have only to divide the iron into excessively small particles to discover that it is really the more combustible of the two, and that it actually takes fire spontaneously and burns like tinder. It is the excessive slowness of the process in the case of a massive piece of iron which disguises its real character.
If Xenophon reports truly, Socrates was misled by not making sufficient allowance for extreme differences of degree and quantity. Anaxagoras held that the sun is a fire, but Socrates rejected this opinion, on the ground