478 THE POPULAR SCIENCE MONTHLY. 



RAIN-MAKING.* 



BY FEENANDO SANFOBD, 



PROFESSOR OF PHYSICS, LELAND STANFORD JUNIOR UNIVERSITY. 



I SHALL ask your attention this evening to the scientific prin- 

 ciples which are involved in the condensation of atmospheric 

 vapor, and to some of the attempts which have been made to pro- 

 duce this condensation by artificial means. 



Since the change from atmospheric vapor to water involves a 

 change of the physical state of the same substance from a gas to 

 a liquid, it is important that we understand clearly the difference 

 between these two physical states. 



Both liquids and gases are undoubtedly made of very small 

 particles called molecules. In a gas these molecules are not held 

 together by any force, but each molecule is a perfectly independ- 

 ent body, free to move in any direction without reference to any 

 other molecule, except as its motion may be interfered with by 

 colliding with another. Under all known conditions these gase- 

 ous molecules are actually in rapid motion, each one moving at 

 its own rate and in its own path, unaffected by any known force 

 except gravitation. Each molecule will, accordingly, move in a 

 straight line until it collides with another molecule. When two 

 molecules collide, their direction of motion will be changed ac- 

 cording to the angle of collision, but on account of their high 

 elasticity they rebound with the same force with which they col- 

 lide, and the sum of their motions will be practically the same as 

 before. Hence, no number of collisions between the molecules 

 themselves will ever bring them to rest. 



If confined within solid walls, they strike against these walls 

 and rebound from them just as they do from each other. In do- 

 ing so each molecule exerts a pressure upon the wall during its 

 time of contact, and the sum of these pressures is the whole pres- 

 sure of the gas upon the walls of its containing vessel. 



These walls are likewise composed of similar molecules, but 

 held together by some unknown force, and it is the surface layer 

 of these molecules which must bear the shock of the molecular 

 bombardment of the gas. Accordingly, the molecules of the 

 solid walls, while not free to be driven about from one place to 

 another, like the gaseous molecules, are nevertheless set in vibra- 

 tion ; and since they can not lie as close together while in vibra- 

 tion as they could at rest, the solid mass of the walls is made to 

 expand. By measuring the amount of this expansion we can de- 



* A lecture given before the students of the Leland Stanford Junior University, March 

 6, 1894. 



