To return to the colder scientific aspect of chemistry.  It has shown us how bodies stand affected to each other through an almost boundless range of combinations.  It has given us a most ingenious theory to account for certain fixed relations in these combinations.  It has successfully eliminated a great number of proximate compounds, more or less stable, from organic structures.  It has invented others which form the basis of long series of well-known composite substances.  In fact, we are perhaps becoming overburdened with our list of proximate principles, demonstrated and hypothetical.

How much nearer have we come to the secret of force than Lully and Geber and the whole crew of juggling alchemists?  We have learned a great deal about the how, what have we learned about the why?

Why does iron rust, while gold remains untarnished, and gold amalgamate, while iron refuses the alliance of mercury?

The alchemists called gold Sol, the sun, and iron Mars, and pleased themselves with fancied relations between these substances and the heavenly bodies, by which they pretended to explain the facts they observed.  Some of their superstitions have lingered in practical medicine to the present day, but chemistry has grown wise enough to confess the fact of absolute ignorance.

What is it that makes common salt crystallize in the form of cubes, and saltpetre in the shape of six-sided prisms?  We see no reason why it should not have been just the other way, salt in prisms and saltpetre in cubes, or why either should take an exact geometrical outline, any more than coagulating albumen.

But although we had given up attempting to explain the essential nature of affinities and of crystalline types, we might have supposed that we had at least fixed the identity of the substances with which we deal, and determined the laws of their combination.  All at once we find that a simple substance changes face, puts off its characteristic qualities and resumes them at will;—­not merely when we liquefy or vaporize a solid, or reverse the process; but that a solid is literally transformed into another solid under our own eyes.  We thought we knew phosphorus.  We warm a portion of it sealed in an empty tube, for about a week.  It has become a brown infusible substance, which does not shine in the dark nor oxidate in the air.  We heat it to 500 F., and it becomes common phosphorus again.  We transmute sulphur in the same singular way.  Nature, you know, gives us carbon in the shape of coal and in that of the diamond.  It is easy to call these changes by the name allotropism, but not the less do they confound our hasty generalizations.