780 Sir J. J. Thomson on the 



there is no charge on the atoms, the dissociation would not 

 give rise to charged ions which could be carried along by the 

 gas, but if the atoms in the molecule were charged the dis- 

 sociation would produce ions, and these being carried along 

 by the gas would produce electrification like that observed 

 when gas bubbles through these liquids. 



The magnitude of the electrification is of the same order 

 whether the liquid through which the gas is babbled is 

 distilled water or a salt or acid solution, though there are in 

 other respects very marked differences between the two 

 cases. As the number of ions in the solution is of quite a 

 different order from that in the water, we conclude that the 

 electrification cannot be due to the unaided diffusion of these 

 ions into the cavities produced by the air-bubbles. 



On the conditions for the existence of a Chemical 

 ComjDouncl and on Valency. 



In order that a chemical compound may exist in a 

 stable form it must satisfy certain conditions : one of 

 these is that its molecules must not exert on other molecules 

 in their neighbourhood attractions large enough to cause 

 the molecules to unite and thus form other systems ; another 

 condition is that the attractions between the atoms in the 

 molecule must be great enough to hold them together in 

 spite of the blows they receive when the molecule collides 

 with other molecules. It depends to a very large extent on 

 the temperature whether these conditions are fulfilled or not. 

 The first condition wall be satisfied if the work required to 

 separate the two molecules whose combination is under con- 

 sideration is small compared with a 0, the energy corre- 

 sponding to one degree of freedom at 0, the absolute 

 temperature of the molecules. The second, if the w r ork 

 required to separate the atoms in the molecule is large 

 compared with the same quantity. Thus at high temperatures 

 we may have systems existing in a stable form which would 

 pass into more complex ones if the temperature w r ere lower,, 

 while on the other hand some molecules would break up at 

 high temperatures though they w r ould be stable at lower 

 ones. Both these conditions are illustrated by a case like 

 that of iodine vapour, whose molecules at a high temperature 

 are monatomic while at a lower one they are diatomic. 

 Thus the question as to whether a substance is, or is not r 

 ' saturated,' involves essentially the question of temperature, 

 and even unsaturated substances can exist if the temperature 

 is not too low. 



