76 



DISCOVERY 



than one form) there is a similar temperature, termed 

 the transition point. Thus in the case of sulphur, at 

 temperatures below 96° the stable modification is 

 rhombic sulphur ; at temperatures above 96°, the 

 stable phase is a different crystalline variety known as 

 monoclinic sulphur. Each of these varieties possesses 

 its own melting point, that of rhombic sulphur being 

 114-5°, s-nd that of the monochnic variety being 120^. 

 The transformation of either form into the other is a 

 reversible process at the transition point, just as the 

 transformation of ice into liquid water and vice versa 

 is a reversible process. The analogy between a melt- 

 ing point and a transition point is, therefore, remark- 

 ably close. There is, however, one important point of 

 difference between them. It is possible to cool a 

 liquid a few degrees below its freezing point without 

 solidification taking place, the liquid being then in 

 what is termed a metastable condition. On the other 

 hand, no one has ever succeeded in heating a solid 

 above its melting point. When we study poly- 

 morphic substances, we find that the transition point 

 is, in general, very easily overstepped in both direc- 

 tions, because the rate of transformation of one form 

 into the other is not great. It is for this reason that 

 it is possible to determine the melting point of rhombic 

 sulphur. If the substance is heated fairly quickly, 

 the transition point (96°) is overstepped, and the melt- 

 ing point (114-5°) realised. 



An interesting example of a substance which is 

 polymorphic is to be found in the case of tin. As 

 usually met with, this metal is a white crystalline solid. 

 When exposed for long periods of time at somewhat 

 low temperatures, the metal loses its familiar white 

 crystalline appearance,, and crumbles to a grey powder. 

 Objects made of this metal, such as old coins or organ 

 pipes in old churches, frequently show signs of what 

 would appear to be corrosion, but what is really 

 transformation of ordinary white tin into the grey 

 variety which is the stable modification at tempera- 

 tures below 18°. Actually, all tin, except in warm 

 summer weather, is in the metastable condition, but as 

 the transformation into the stable grey powder is so 

 sluggish, objects made of this metal can be preserved 

 for long periods. 



Reversibility of Transformation 



Substances such as sulphur and tin, whose behaviour 

 is such that the transformation of one form into the 

 other is a reversible process at the transition point, 

 are termed enantiotropic. There is another class of 

 substances which differs from these, in that one form 

 is always less stable than the other ; the transformation 

 of that form into the other proceeds in one direction 

 only. Phosphorus is an example of this class of 

 substances. The two best-known forms of phosphorus 



are the white and the red varieties. Now white 

 phosphorus is always metastable with respect to red. 

 If sticks of white phosphorus are stored in a bottle 

 containing water and exposed to light for long periods, 

 patches of the red variety gradually make their 

 appearance. There is only one way of transforming 

 red phosphorus into the white form, and that is by 

 vaporising the substance and quickly cooling the 

 vapour. WHien a solid substance is produced from a 

 liquid or vapour it does not, in general, assume the 

 most stable configuration all at once, but first takes 

 up an unstable one, which may remain for a long time 

 before the most stable form is reached. It is in this 

 way that substances such as white phosphorus can be 

 prepared. Substances of this kind are termed mono- 

 tropic. In the case of enantiotropic substances the 

 transition point lies between the melting point of the 

 two forms ; in the case of monotropic substances it is 

 probable that the melting points of both forms are 

 below the transition point. 



In the case of both classes of polymorphic sub- 

 stances, the less stable form has always the greater 

 vapour pressure and the greater solubUity. This 

 general phenomenon has been found experimentally, 

 but it is only what would necessarily be expected on 

 theoretical grounds. It is not always an easy matter 

 to decide whether two specimens of a substance, which 

 appear to differ in some property, are truly poly- 

 morphic. The experimental difficulties of determining 

 the existence or otherwise of a transition point are 

 frequently verj' considerable. An interesting example 

 is to be found in the case of mercuric oxide. This 

 substance can be obtained in either a yellow or red 

 form, depending upon the method of preparation. 

 Determinations of solubility carried out by electric 

 measurements appear to indicate a slight but distinct 

 difference between the two forms. On this ground, 

 some have concluded that mercuric oxide is poly- 

 morphic. Ostwald has, however, pointed out that 

 the solubility determinations do not admit of any 

 conclusion being drawn. It is well known that the 

 solubility of sparingly soluble substances depends 

 upon the size of the particles, that of very small 

 particles being greater than that of larger ones ; just 

 as the vapour pressure of small drops is greater than 

 that of larger ones, as was pointed out by Lord Kelvin 

 many years ago. The difference between yellow and 

 red mercuric oxide may be entirely due to the difference 

 in size of the particles. 



The Liquid State of Aggregation 



The investigation of the liquid state of aggregation 

 from the point of view of pol^nnorphism has yielded 

 many results of interest. Certain organic compounds 

 when heated melt sharply at definite temperatures to 



