AXXIVEESAEY ADDEESS OF THE TEESIEENT. 69 



We have already seen that every alteration in the temperature or 

 other conditions which surround a crystal leads to striking modifi- 

 cations of molecular structure, which are at once revealed by the 

 delicate tests of optical analysis. So sensitive, indeed, are some 

 crystals to the action of external forces, that even the passage of the 

 light-waves through their substance leads to permanent molecular 

 rearrangements which are evidenced by marked changes in colour, 

 translucency, and other properties. 



Many minerals have their atoms so arranged that the action of 

 external forces causes them to fall readily into new combinations. In 

 this way we have brought about such paramorphic changes as that 

 of aragonite into calcite, and angite into hornblende. Excessively 

 slight manifestations of force are sometimes sufficient to induce 

 these paramorphic changes. 



But the most significant fact of all is that every crystal possesses 

 certain peculiarities of molecular structure, and as the result of this 

 internal " organization " it responds in a definite manner to the action 

 of various external forces, undergoing in this way well-marked 

 series of physical and chemical changes without losing its identity. 

 As the final result of such successive changes, however, the 

 bonds which hold the " organized " structures together are gradually 

 weakened, and at last break down altogether. In this way the 

 separate existence of the mineral comes to an end ; but the materials 

 of which it was composed, resolving themselves into new compounds, 

 may go to build up the substance of other " organized " structures, 

 j^eed I point out that in all these respects minerals behave exactly 

 like plants and animals? 



But in the case of plants and animals changes such as these, 

 which are the direct outcome of external forces acting on a special 

 organization, are called Physiological, and I know of no valid reason 

 why the same term should not be employed in the case of minerals. 

 It is true that the accomplishment of the cycles of change in 

 minerals often requires periods of time of enormous duration, and 

 that during incalculable intervals they may appear to be wholly 

 suspended ; but in these respects the " life " of a mineral differs from 

 that of a plant in just the same manner as the latter does from the 

 life of an animal. 



I must ask your attention for a few moments to these peculiarities 

 of internal organization in minerals, and to the way in which the 

 various physical and chemical forces act and react upon them in con- 

 sequence of their special organization. 



