SC1KNCE AND MAN. 601 
the ocean from which it came. Up to the point where 
condensation begins, an amount of heat exactly equivalent 
to the molecular work of vaporization and the mechanical 
work of lifting the vapor to the mountain-tops has disap- 
peared from the universe. What is the gain corresponding 
to this loss? It will seem when mentioned to be expressed 
in a foreign currency. The loss is a loss of heat; the gain 
is a gain of distance, both as regards masses and molecules 
Water which was formerly at the sea-level has been lifted 
to a position from which it can fall; molecules which have 
been locked together as a liquid are now separate as vapor 
which can recompense. After condensation gravity comes 
into effectual play, pulling the showers down upon the 
hills, and the rivers thus created through their gorges to 
the sea. Every raindrop which smites the mountain pro- 
duces its definite amount of heat; every river in its course 
develops heat by the clash of its cataracts and the friction 
of its bed. In the act of condensation, moreover, the 
molecular work of vaporization is accurately reversed. 
Compare, then, the primitive loss of solar warmth with 
the heat generated by the condensation of the vapor, and 
by the subsequent fall of the water from cloud to sea. 
They are mathematically equal to each other. No particle 
of vapor was formed and lifted without being paid for in 
the currency of solar heat; no particle returns as water to 
the sea without the exact quantitative restitution of that 
heat. There is nothing gratuitous in physical nature, no 
expenditure without equivalent gain, no gain without 
equivalent expenditure. With inexorable constancy the 
one accompanies the other, leaving no nook or crevice 
between them for spontaneity to mingle with the pure and 
necessary play of natural force. Has this uniformity of 
nature ever been broken? The reply is: "Not to the 
knowledge of science." 
What has been here stated regarding heat and gravity 
applies to the whole of inorganic nature. Let us take an 
illustration from chemistry. The metal zinc may be burned 
in oxygen, a perfectly definite amount of heat being pro- 
duced by the combustion of a given weight of the metal. 
But zinc may also be burned in a liquid which contains a 
supply of oxygen in water, for example. It does not in 
this case produce flame or tire, but it does produce heat 
which is capable of accurate measurement. But the heat 
