The one chemical process with which we are familiar all our lives, but which we never think of as such, is fire.  Here on our own hearthstones goes on this wonderful spectacular and beneficent transformation of matter and energy, and yet we are grown so familiar with it that it moves us not.  We can describe combustion in terms of chemistry, just as we can describe the life-processes in similar terms, yet the mystery is no more cleared up in the one case than in the other.  Indeed, it seems to me that next to the mystery of life is the mystery of fire.  The oxidizing processes are identical, only one is a building up or integrating process, and the other is a pulling down or disintegrating process.  More than that, we can evoke fire any time, by both mechanical and chemical means, from the combustible matter about us; but we cannot evoke life.  The equivalents of life do not slumber in our tools as do the equivalents of fire.  Hence life is the deeper mystery.  The ancients thought of a spirit of fire as they did of a spirit of health and of disease, and of good and bad spirits all about them, and as we think of a spirit of life, or of a creative life principle.  Are we as wide of the mark as they were?  So think many earnest students of living things.  When we do not have to pass the torch of life along, but can kindle it in our laboratories, then this charge will assume a different aspect.

III

Nature works with such simple means!  A little more or a little less of this or that, and behold the difference!  A little more or a little less heat, and the face of the world is changed.

    “And the little more, and how much it is,
    And the little less, and what worlds away!”

At one temperature water is solid, at another it is fluid, at another it is a visible vapor, at a still higher it is an invisible vapor that burns like a flame.  All possible shades of color lurk in a colorless ray of light.  A little more or a little less heat makes all the difference between a nebula and a sun, and between a sun and a planet.  At one degree of heat the elements are dissociated; at a lower degree they are united.  At one point in the scale of temperatures life appears; at another it disappears.  With heat enough the earth would melt like a snowball in a furnace, with still more it would become a vapor and float away like a cloud.  More or less heat only makes the difference between the fluidity of water and the solidity of the rocks that it beats against, or of the banks that hold it.

The physical history of the universe is written in terms of heat and motion.  Astronomy is the story of cooling suns and worlds.  At a low enough temperature all chemical activity ceases.  In our own experience we find that frost will blister like flame.  In the one case heat passes into the tissues so quickly and in such quantity that a blister ensues; in the other, heat is abstracted so quickly and in such quantity that a like effect is produced.  In one sense, life is a thermal phenomenon; so are all conditions of fluids and solids thermal phenomena.