284 THE POPULAR SCIENCE MONTHLY. 



in gases, but by no means in the same proportion. The same amount 

 of motion takes about ten times as long to subside in water as in air, 

 as you will see by what takes place when I stir these two jars, one 

 containing water and the other air. There is still less difference be- 

 tween the rates at which a rise of temperature is propagated through 

 a liquid and through a gas. 



In solids the molecules are still in motion, but their motions are 

 confined within very narrow limits. Hence, the diffusion of matter 

 does not take place in solid bodies, though that of motion and heat 

 takes place very freely. Nevertheless, certain liquids can diffuse 

 through colloid solids, such as jelly and gum, and hydrogen can make 

 its way through iron and palladium. 



We have no time to do more than mention that most wonderful 

 molecular motion which is called electrolysis. Here is an electric 

 current passing through acidulated water, and causing oxygen to 

 appear at one electrode, and hydrogen at the other. In the space 

 between, the water is perfectly calm, and yet two opposite currents 

 of oxygen and of hydrogen must be passing through it. The physical 

 theory of this process has been studied by Clausius, who has given 

 reasons for asserting that in ordinary water the molecules are not only 

 moving, but every now and then striking each other with such violence 

 that the oxygen and hydrogen of the molecules part company, and 

 dance about through the crowd, seeking partners which have become 

 dissociated in the same way. In ordinary water these exchanges pro- 

 duce, on the whole, no observable effect, but no sooner does the elec- 

 tromotive force begin to act, than it exerts its guiding influence on 

 the unattached molecules, and bends the course of each toward its 

 proper electrode, till the moment when, meeting with an unappro- 

 priated molecule of the opposite kind, it enters again into a more or 

 less permanent union with it till it is again dissociated hy another 

 shock. Electrolysis, therefore, is a kind of diffusion assisted by elec- 

 tromotive force. 



Another branch of molecular science is that which relates to the 

 exchange of molecules between a liquid and a gas. It includes the 

 theory of evaporation and condensation, in which the gas in ques- 

 tion is the vapor of the liquid, and also the theory of the absorption 

 of a gas by a liquid of a different substance. The researches of Dr. 

 Andrews on the relations between the liquid and the gaseous state 

 have shown us that, though the statements in our own elementary text- 

 books may be so neatly expressed that they appear almost self-evident, 

 their true interpretation may involve some principle so profound that, 

 till the right man has laid hold of it, no one ever suspects that any 

 thing is left to be discovered. 



These, then, are, some of the fields from which the data of molec- 

 ular science are gathered. "We may divide the ultimate results into 

 three ranks, according to the completeness of our knowledge of them. 



