Variable Properties of Matter.

I have enumerated some of the few properties of matter, which are manifested in exactly the same manner by all substances, whatever be their differences of chemical or physical constitution. But by far the greater number of qualities vary in degree; substances are more or less dense, more or less transparent, more or less compressible, more or less magnetic, and so on. One very common result of the progress of science is to show that qualities supposed to be entirely absent from many substances are present only in so low a degree of intensity that the means of detection were insufficient. Newton believed that most bodies were not affected by the magnet at all; Faraday and Tyndall have rendered it very doubtful whether any substance whatever is wholly non-magnetic, including under that term diamagnetic properties. We are rapidly learning to believe that there are no substances absolutely opaque, or non-conducting, non-electric, non-elastic, non-viscous, non-compressible, insoluble, infusible, or non-volatile. All tends to become a matter of degree, or sometimes of direction. There may be some substances oppositely affected to others, as ferro-magnetic substances are oppositely affected to diamagnetics, or as substances which contract by heat are opposed to those which expand; but the tendency is certainly for every affection of one kind of matter to be represented by something similar in other kinds. On this account one of Newton's rules of philosophizing seems quite to lose all validity; he said, Those qualities of bodies which are not capable of being heightened and remitted, and which are found in all bodies on which experiment can be made, must be considered as universal qualities of all bodies.' As far as I can see, the contrary is more probable, namely,

that qualities variable in degree will be found in every substance in a greater or less degree.

It is highly remarkable that Newton, whose method of investigation was logically perfect, seemed incapable of generalizing and describing his own procedure. His celebrated Rules of reasoning in Philosophy,' described at the commencement of the third book of the Principia,' are of very questionable truth, and still more questionable value.

Extreme Instances of Properties.

Although, as we have seen, substances usually differ only in degree as regards their physical properties, great interest may attach to particular substances which manifest a property in a conspicuous and intense manner. Every branch of physical science has usually been developed from the attention forcibly drawn to some singular substance. Just as the loadstone disclosed magnetism and amber frictional electricity, so did Iceland spar point out the existence of double refraction, and sulphate of quinine the phenomenon of fluorescence. When one such startling instance has drawn the attention of the scientific world, numerous less remarkable cases of the phenomenon will soon be detected, and it will probably prove that the property in question is actually universal to all matter. Nevertheless, the extreme instances retain their interest, partly in a historical point of view, partly because they furnish the most convenient substances for experiment.

Francis Bacon was fully aware of the value of such examples, which he called Ostensive Instances or Lightgiving, Free or Predominant Instances. They are those,' he says, which show the nature under investigation naked, in an exalted condition, or in the highest degree

of power; freed from impediments, or at least by its strength predominating over and suppressing them i.' He mentions quicksilver as an ostensive instance of weight or density, thinking it not much less dense than gold, and more remarkable than gold as joining density to liquidity. The magnet is mentioned as an ostensive instance of attraction. It would not be very easy to distinguish clearly between these ostensive instances and those which he calls Instantiae Monodicae, or Irregulares, or Heteroclitae, under which he places whatever is extravagant in its properties or magnitude, or exhibits least similarity to other things, such as the sun and moon among the heavenly bodies, the elephant among animals, the letter s among letters, or the magnet among stones 1.

In optical science great use has been made of the high dispersive power of the transparent compounds of lead, that is, the power of giving a long spectrum (vol. i. p. 32). Dolland having noticed the peculiar dispersive power of lenses made of flint-glass employed them to produce an achromatic arrangement. The element strontium presents a contrast to lead in this respect, being characterized by a remarkably low dispersive power; but I am not aware that this property has yet been turned to account.

Compounds of lead have both a high dispersive and a high refractive index, and in the latter respect they proved very useful to Faraday. Having spent much labour in preparing various kinds of optical glass, Faraday happened to form a compound of lead, silica, and boracic acid, now known as heavy glass, which possessed an intensely high refracting power. Many years afterwards in attempting to discover the action of magnetism upon light he failed to detect any effect, as has been i Novum Organum,' bk. II. Aphorism 24.

k Ibid. Aphorism 25.

1 Ibid. Aphorism 28.

already mentioned (vol. ii. p. 235), until he happened to test a piece of the heavy glass. The peculiar refractive power of this medium caused the magnetic strain to be apparent, and the rotation of the plane of polarization was discovered.

In almost every other 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 polarization of light, from 15 to 17 times that of quartz. In electric experiments copper is employed for its high conducting powers and exceedingly low magnetic properties; iron is of course essential for its enormous and almost anomalous 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 m. In regard to magnecrystallic action the mineral cyanite is highly remarkable, being so powerfully affected by the earth's magnetism, that when delicately suspended, it will assume 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 extreme that probably one half of the whole number of stars in the heavens have a yellow tinge in consequence. It is highly remarkable that water, though the most common of all fluids, is distinguished in almost every mPhilosophical Transactions,' (1856), vol. cxlvi. p. 246.

Of all known

respect by the most marked qualities. 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". In proportion to its density it has a far higher surface tension than any other substance, being in fact 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, equally with those of the opposite extreme. 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. 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 at once 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 P. The compounds of strontium have the lowest dispersive powers on light. It will be 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 always bear in mind that phenomena which are in reality of a closely similar or even identical nature,

n Philosophical Magazine,' 4th Series, January 1870, vol. xxxix. p. 2. • Novum Organum,' bk. II. Aphorism 25.

Faraday's Experimental Researches in Chemistry and Physics,'