[Footnote 119: Connection of Phys. Sciences, § 14, p. 125.]
[Footnote 120: Phys. Geog. I, ch. xv. pp. 288, 289.]
In the language of the older treatise on science, glass is said to be transparent: gold, coal, etc., opaque, that is, incapable of transmitting light. But there is no substance known to modern discovery which, if sufficiently attenuated, is not capable of being seen through. Opacity, therefore, has no real existence as a quality of matter; it depends only on condition and circumstances. Hardness or softness in like manner, are easily separable from the substance of matter. Clay in its natural state is soft, apply heat to it and it becomes hard; wax is naturally hard, but becomes soft and ductile when warmed. Thus our knowledge of the internal constitution of material substance, through the medium of its external qualities, is in the highest degree uncertain, variable, and often erroneous. For there is not one of those external notes or Marx, which we call qualities, which cannot be changed or modified in such a way as seriously to derange the accuracy of our observations. Enough of accuracy has been secured for the purposes of our daily life; but, like the senses, our knowledge of the relation of quality to substance was never intended to carry us through the boneless field of knowledge, or enable us to pronounce with certainty regarding the nature, the difference, or the identity of substance, merely from the indications given us by its apparent qualities. These are truly accidents; things which do not affect the essence of matter; but connected with it in an evanescent way, liable to sudden change, and totally baffling our attempts to establish any certain criterion of substance by means of our observations on its qualities.
Recent observations in chemistry have still further demonstrated the impossibility of arriving at any knowledge of the internal structure of matter from its appearances. The delicate tests invented by chemists, in order to detect the difference between substances which appear to every human sense the same, though they effect their purpose with marvelous ingenuity, yet fail in indicating the ultimate reason for their efficiency.
Thus syrup extracted from the sugar-cane, or from plants yielding similar sugar, looks in every respect the same as that extracted from the juice of the grape. The refinements of modern chemistry, however, have pointed out several tests to distinguish one from the other. [Footnote 121] And in a beam of polarized light there is provided a test as subtle as any contributed by the aid of chemistry. In the instance of cane sugar, the plane of polarization revolves to the right; in grape sugar, it revolves to the left. Of this subtle agent, Mrs. Somerville remarks, when stating this interesting fact, that '"it surpasses the power even of chemical analysis in giving certain and direct evidence of the similarity or difference existing in the molecular constitution of bodies, as well as of the permanency of that constitution, or of the fluctuations to which it may be liable." [Footnote 122] The same delicate test of polarization enables us to distinguish reflected light, such as the moon's, from the light which issues from a self-luminous body, like Sirius. But in all these instances, the ultimate rationale of its indications still remains veiled in impenetrable darkness; and with it, any knowledge of the internal substance of matter.
[Footnote 121: Brande's Lectures on Organic Chemistry, p. 153.]was it
[Footnote 122: Connexion of Physical Sciences, § xxii, p. 214.]
It is, however, in the mysterious facts to which chemists have given the names of Isomorphism, Isomerism, and Allotropism, that we perceive the most direct and remarkable contribution of modern scientific research to the defence of Catholic revelation. Chemistry enables us to penetrate further than any other science into the secret operations of Nature; and strange insight has been thus obtained into the identity of substance under two or more external appearances; and of the existence of two or more substances of distinct character under identical appearances. A few words will not be idly devoted to a description of these terms, and of the results associated with them.
Isomorphism expresses the phenomenon in crystallization established by Gay Lussac and Mitscherlich, of different compounds assuming the same crystalline form. The generally received law of this process had hitherto been, that the same substances invariably crystallize in forms belonging to one system, different substances, in forms belonging to another. Cases had indeed been observed, before the discovery of Isomorphism, in which the same element had been seen to crystallize in two forms, belonging to different systems, not geometrically connected. Sulphur, for instance, crystallizing from its solution in the bisulphuret of carbon, assumes a geometrically different crystalline form from sulphur when melted by heat, and allowed to consolidate as it cools. But these and a few other similar cases had been explained as depending on a different arrangement of the particles, due most probably to a difference in the temperature during the operation. They were not thought to interfere with the general law of the same substance always assuming the same crystalline form. The two eminent philosophers just mentioned ascertained beyond a doubt that, in many instances, compound substances, in the process of crystallizing, assume the same or a cognate form, though their elements are totally different. Thus chloride of sodium (sea salt), sulphate of alumina and potash (alum), and many other compound substances equally dissimilar, crystallize in the form of the cube and its congeners. Other crystalline forms also are found to be common to many differently constituted compounds. "To these groups of analogous elements," says Professor Gregory, from whose work, On Inorganic Chemistry, we have abridged this account, "the name of Isomorphous groups has been given, as there is every reason to believe that as elements they possess the same form; and the phenomena of identical form in compounds of different but analogous composition, have received the name of Isomorphism. Two elements [{376}] are said to be isomorphous, which either crystallize in the same form, or may be substituted for each other in their compounds, equivalent for equivalent (the other elements remaining unchanged), without affecting the form of the compound. We can hardly doubt that not only the salt, but the acids are really isomorphous, and would be found so if we could obtain them all in crystals; and we have the same reason to conclude that the elements of these acids are also isomorphous; that arsenic and phosphorus, sulphur and selenium, for example, crystallize in the same form." [Footnote 123]
[Footnote 123: Inorganic Chemistry, Ed. 1853; pp. 38 et seq. ]
The converse of this phenomenon is also included among the discoveries of modern science; the same substance is sometimes observed to crystallize in two different forms not geometrically allied; and the occurrence of this new exception to the received law of crystallization is called Dimorphism.