SCIENCE AND MAN" 359 



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 with- 

 out 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 meUl 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 fire, but it does produce 

 heat which is capable of accurate measurement. But the 

 heat of zinc burned in water falls short of that produced 

 in pure oxygen, the reason being that to obtain its oxy- 

 gen from the water the zinc must first dislodge the hy- 

 drogen. It is in the performance of this molecular work 

 that the missing heat is absorbed. Mix the liberated 



