632 SCIENCE PROGRESS 



of vibration of every electron in a compound molecule we are 

 able nevertheless to make some progress in understanding the 

 mechanism of chemical change by following a more generalised 

 train of reasoning. We have seen from our previous con- 

 siderations that a molecule may decompose or react with 

 another provided the nuclei become sufficiently widely separ- 

 ated from one another. To bring about this incipient dissocia- 

 tion of the molecule we must add energy to it in the form of 

 heat, for it is well known that many substances dissociate or 

 decompose when their temperature is raised. In other words, 

 we can think of a molecule in the non-reactive state as con- 

 taining a certain amount of energy which we shall call the 

 mean or average internal energy. We may likewise conceive 

 of this energy being increased by absorption of heat until it 

 reaches a stage, which we may call the critical stage, at which 

 the molecule becomes chemically reactive. By carrying out 

 certain measurements on the speed at which a reaction pro- 

 ceeds at different temperatures it is possible to calculate what 

 the critical energy is for a given kind of molecule, and in this 

 way compare the relative reactivities of different molecular 

 species in numerical terms. This kind of investigation repre- 

 sents a new branch of the subject which has been followed 

 only within the last few years. It is too early yet to properly 

 assess its value A very wide field of research opens in the 

 direction of correlating the structure of molecules with the 

 critical energy which has to be added to them to make the 

 molecule reactive. 



The Solid State of Matter 



The characteristic feature which distinguishes the solid 

 state from the other two is the existence of crystalline form. 

 (Amorphous solids, such as glass, are grouped as liquids of 

 very high viscosity.) A true solid separates from solution or 

 from vapour in the form of crystals, having definite and 

 characteristic properties, such as the melting point for ex- 

 ample. (Amorphous solids or supercooled liquids possess no 

 definite melting point.) Crystalline form exhibits many 

 varieties, into the details of which it is unnecessary to enter, 

 as our present considerations deal only with one or two types 

 belonging to the regular or cubic system. 



For many years the solid state presented greater theoretical 



