SCIENCE AND MAN. 341 



companies loss, no item varying in the slightest degree 

 without an absolutely equal change of some other item 

 in the opposite direction. 



The sun warms the tropical ocean, converting a 

 portion of its liquid into vapour, which rises in the air 

 and is recondensed on mountain heights, returning in 

 rivers to the ocean from which it came. Up to the 

 point where condensation begins, an amount of heat 

 exactly equivalent to the molecular work of vaporisa- 

 tion and the mechanical work of lifting the vapour to 

 the mountain-tops has disappeared from the universe. 

 What is the gain corresponding to this loss? It will 

 seem when mentioned to be expressed in a foreign cur- 

 rency. 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 vapour which can recondense. 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 produces 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 vapori- 

 sation is accurately reversed. Compare, then, the 

 primitive loss of solar warmth with the heat generated 

 by the condensation of the vapour, and by the subse- 

 quent fall of the water from cloud to sea. They are 

 mathematically equal to each other. No particle of 

 vapour 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 



