50 CRYSTALS AND LIVING UNITS 



because at about 18° C all crystals disappear, and the fluid would 

 be sterilised. 



Thus, from the point of view of crystallisation, there are two 

 classes of fluids ; one (a) in which, under appropriate conditions, 

 there may be a spontaneous generation of crystals, as well as a 

 spread of the process induced by inoculation ; and another (6) 

 where the latter process only seems possible and in which genera- 

 tion has never been known to occur. These two classes of fluids, 

 of another order, have, of course, been long recognised also by 

 those who believe in the " spontaneous generation " of microbes. 



The ' germ ' theory of the origin of crystals in supersaturated 

 solutions has, therefore, not only been in existence, but has been 

 overthrown. This has been possible, however, only because it has 

 been more easy to show that a given set of conditions are inimical 

 to the existence of a crystal, than it has yet been to induce people 

 to believe that any given set of suitable experimental conditions, 

 yielding positive results, are incompatible with the pre-existence of 

 germs of living matter. 



The analogy between the two problems as to the possible origin 

 of crystals and organisms de novo in solutions, was rendered much 

 more obvious by the discovery of the late Prof. Graham that, when 

 dissolved, the saline substance does not remain as such in solution 

 — ^but that the acid and the base exist separately, and are separable 

 by a process of dialysis. When crystallisation occurs, therefore, we 

 have a combination of molecules taking place similar to, though 

 much simpler than, what may be presumed to take place in the 

 genesis of a speck of living matter. 



So far as evidence derived from microscopical examination can 

 be adduced, however, it is able to speak no more decisively con- 

 cerning the de novo origin of crystals, than concerning the de novo 

 origin of organisms. In the elucidation of this point the valuable, 

 though insufficiently known, observations of Geo. Rainey^ come 

 most opportunely to our aid. In ordinary cases, it is difficult to 

 watch satisfactorily with the microscope the first stage in the 

 appearance of crystals in solutions containing crystallisable matter 

 owing to the rapidity with which their growth takes place. This 

 is one point in which crystals are strikingly different from 

 organisms. The slower growth of organisms is, however, as 

 Graham pointed out, quite in accordance with the general slowness 

 ' " On the Mode of Formation of the Shell of Animals," etc. London, 1858, p. 9. 



