824 Professor William J. Pope [April 15, 



retained for many hours at a temperature far below the norraal melting 

 point of the compound. But on inoculating the liquid with a trace of 

 crystalline benzophenone crystallisation immediately commences and 

 rapidly becomes complete. The introduction of a small particle of 

 crystalline or arranged material into the liquid mass provides a nucleus 

 upon which the molecules are able to deposit themselves in a similar 

 crystalline arrangement; the process thus started quickly becomes propa- 

 gated throughout the entire mass. The lack of spontaneity in the pro- 

 cess of crysfcalhsation leads occasionally to quite unexpected results. 

 Thus, tetrahydroquinaldine has been known for many years, and has 

 been prepared by numbers of chemists. It has always been obtained 

 as a liquid, and has never been supposed capal)le of existing in the 

 crystalline state at ordinary temperatures : even when cooled in liquid 

 air it merely becomes a thick resin, and does not crystallise. But on 

 dissolving a few drops of it in a little light petroleum and cooling the 

 solution thus obtained in hquid air, the tetrahydroquinaldine crystal- 

 lises out : on transferring a trace of the crystalline material obtained 

 to the liquid substance at the ordinary temperature, the liquid mass is 

 seen to immediately crystallise. This well-known substance, hitherto 

 known only in the liquid state at ordinary temperatures, really exists 

 in a more stable condition as a crystalline solid. 



Many substances are capable of crystallising in two or more dis- 

 tinct crystalline forms of which one is, in general, the more stable at 

 any particular temperature. The physical properties of the several 

 crystalline modifications of any one substance are quite distinct and 

 characteristic for the particular crystalline form and, in many in- 

 stances, even the colours of the several modifications are different. 

 An example of this is afforded by pouring boiling water into a beaker 

 coated with cuprous mercuric iodide ; the brilliant scarlet crystalline 

 form stable at ordinary temperatures, when heated in this way, becomes 

 converted into another crystalline modification which is nearly black. 

 The change is a reversible one, and the differences between the pro- 

 perties of the two crystalline modifications are to be attributed to differ- 

 ences in the mode of arrangement of the molecules in the two cases ; 

 the two modifications, in fact, possess different crystalHne structures. 



Although vast numbers of observations, such as the preceding, 

 lead to the conclusion that crystals are arranged structures, it is not 

 essential that the crystal should be a solid substance ; during recent 

 years large numbers of crystalline liquids have been discovered. On 

 allowing melted cholesteryl chloride to cool rapidly a brilliant display 

 of interference colours is seen owing to the particles of the substance 

 assuming crystalline or orderly arrangement whilst still retaining the 

 liquid condition. 



Having very briefly reviewed some of the many reasons for con- 

 cluding that crystals are structured edifices, the nature of the archi- 

 tecture which they exhibit may now be considered. All the properties 

 of crystalline solids harmonise with one simple assumption as to the 



