PHYSICS, BIOLOGY, AND PSYCHOLOGY. 148 



crystal. By increasing the external pressure or adding 

 heat, we can cause a crystal of ice to waste away by melt- 

 ing. If, however, we remove the pressure or the heat, 

 the crystal reforms and grows to its former size. We 

 can also, with proper precautions, cool water to below 

 the freezing-point without any ice forming. But if to 

 the supercooled water we add the smallest crystal of 

 ice, it rapidly grows into a larger crystal, just as the 

 germ of an organism grows. The molecules of water 

 possess the property of attracting one another in such a 

 way as to produce mutual orientation or arrangement, 

 in which they take up more space than when they were 

 present as a mere mobile crowd in the liquid state ; and 

 in the starting of the process of orientation some initial 

 hindrance has to be overcome, so that crystallisation 

 occurs far more readily if it is given a start. We must 

 assume that each molecule possesses the property of so 

 attracting each other molecule as to produce the mutual 

 orientation if there is no hindrance from pressure or 

 from the molecular agitation due to heat, or from other 

 causes. 



An organism maintains itself through a balance 

 between constant loss and gain, whereas the crystal of 

 water seems at first sight not to change except by 

 growth or melting away. When we look closer, however, 

 we find that the crystal has a vapour pressure. It is 

 therefore constantly giving off, and must be equally 

 constantly taking up, water-molecules from its en- 

 vironment. Hence in this respect also it resembles an 

 organism. 



Where the resemblance fails is that the arrangement 

 of the molecules in the crystal is mere repetition, whereas 



