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 remarkable that Newton whose method of investigation was logically perfect, seemed incapable of generalising 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 questionable truth, and still more questionable value.
Extreme Instances of Properties.
Although substances usually differ only in degree, 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 show 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 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 Light-giving, Free and Predominant Instances. “They are those,” he says,7 “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.” 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 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.8
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 (p. 432). Dollond, having noticed this peculiar dispersive power in lenses made of flint glass, employed them to produce an achromatic arrangement. The element strontium presents a contrast to lead in this respect, being characterised 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 already mentioned, (p. 588), 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 polarisation was discovered.