114 CHEMICAL DISCOVERY AND INVENTION 



standard conditions, that is at C., and under a pressure equal 

 to 760 millimetres of mercury, contains 

 2-7 X 10 19 molecules. The highest attain- 

 able vacuum still contains many millions 

 FlG 46 ' per cubic centimetre. 



1 SQUARE CENTIMETRE But the kinetic theory of gases teaches 



contains 10 millimetres on us that in a gas all the molecules are con- 

 each side, and therefore ,1 , , i , 



100 square millimetres in stantly in motion, moving in straight 



contains loweSS^ lines, and frequently striking one against 



metres - another and against the walls of the 



containing vessel, and SQ altering their direction. In liquids 



there is reason to believe that the molecules move but less 



actively, and clusters of them move in company, while in the 



solid state the molecules, though not stationary, vibrate more or 



less rapidly about a mean position, the vibration corresponding 



to what is called their temperature. 



It would be easy to confound the reader with large figures 

 if it were attempted to express the velocity at which molecules 

 travel in a gas, or the mass of an atom of hydrogen or oxygen 

 and so forth, but little would be gained. It is only necessary to 

 remember that gas molecules are far too small to be visible with 

 the aid of any known instrument, that they move with great 

 speed and they collide very frequently. The space between one 

 collision and the next is called the free path of a molecule, and 

 though this varies according to circumstances the mean free 

 path can be calculated. In its original form the kinetic theory 

 was not concerned with the form or nature of the molecule itself 

 or of the atoms of which it is composed. But within the last 

 twenty years the experimental researches, especially of Sir 

 Joseph J. Thomson, on the effects of the electric discharge 

 through attenuated gases have supplied information of the 

 most unexpected and startling kind, which may be regarded as 

 giving to physicists and chemists alike an entirely new point of 

 view as to the ultimate constitution of matter. 



Soon after the improvement of the induction coil by Ruhm- 

 korff some seventy years ago, experiments on the production of 

 sparks in air and other gases led to the discovery of the beautiful 

 luminous effects which are produced when the discharge passes 

 through gases in an attenuated state. It was discovered, among 

 other things, that the colour and appearance of the light depend 

 not on the substance of the electrodes, but on the nature of the 



