meeting in the air and each going through the centre of the other, or the fourth dimension, or almost any other impossible thing, from the point of view of tangible bodies, will serve as well as anything.  The atoms seem to get inside of one another, to jump down one another’s throats, and to suffer a complete transformation.  Yet we know that they do not; oxygen is still oxygen, and carbon still carbon, amid all the strange partnerships entered into, and all the disguises assumed.  We can easily evoke hydrogen and oxygen from water, but just how their molecules unite, how they interpenetrate and are lost in one another, it is impossible for us to conceive.

We cannot visualize a chemical combination because we have no experience upon which to found it.  It is so fundamentally unlike a mechanical mixture that even our imagination can give us no clew to it.  It is thinkable that the particles of two or more substances however fine, mechanically mixed, could be seen and recognized if sufficiently magnified; but in a chemical combination, say like iron sulphide, no amount of magnification could reveal the two elements of iron and sulphur.  They no longer exist.  A third substance unlike either has taken their place.

We extract aluminum from clay, but no conceivable power of vision could reveal to us that metal in the clay.  It is there only potentially.  In a chemical combination the different substances interpenetrate and are lost in one another:  they are not mechanically separable nor individually distinguishable.  The iron in the red corpuscles of the blood is not the metal we know, but one of its many chemical disguises.  Indeed it seems as if what we call the ultimate particles of matter did not belong to the visible order and hence were incapable of magnification.

That mysterious force, chemical affinity, is the true and original magic.  That two substances should cleave to each other and absorb each other and produce a third totally unlike either is one of the profound mysteries of science.  Of the nature of the change that takes place, I say, we can form no image.  Chemical force is selective; it is not promiscuous and indiscriminate like gravity, but specific and individual.  Nearly all the elements have their preferences and they will choose no other.  Oxygen comes the nearest to being a free lover among the elements, but its power of choice is limited.

Science conceives of all matter as grained or discrete, like a bag of shot, or a pile of sand.  Matter does not occupy space continuously, not even in the hardest substances, such as the diamond; there is space, molecular space, between the particles.  A rifle bullet whizzing past is no more a continuous body than is a flock of birds wheeling and swooping in the air.  Air spaces separate the birds, and molecular spaces separate the molecules of the bullet.  Of course it is unthinkable that indivisible particles of matter can occupy space and have dimensions.  But science goes upon this hypothesis, and the hypothesis proves itself.