MODES OF ORIGIN COMPARED 49 



pretty quickly. In the case of betol the most favourable tempera- 

 ture — what in biological language would be called the ' optimum 

 temperature * for the spontaneous generation of crystals — is about 

 10° C. But above and below this the phenomena progressively 

 diminish, so that above 25° or below —5° the betol can be kept for 

 a long time in the liquid state. ... In addition, Tammann dis- 

 covered that small quantities of foreign bodies, soluble or even 

 insoluble (such as rock crystal or glass) suffice greatly to modify the 

 number of germs — sometimes increasing and sometimes diminishing 

 them." 



Now comes a statement which is rather surprising: "But the 

 temperatures most favourable for the generation of crystals are not 

 the most favourable for their rapid growth ; the optimum tempera- 

 ture for growth is notably higher than that for generation." Thus, 

 expose tubes containing liquid betol to about 10° for several minutes, 

 and the fluid will still appear perfectly limpid, the newly-formed 

 crystalline germs being so very minute as to be invisible. Now, 

 as though dealing with a culture of microbes, expose these same 

 tubes to an incubating temperature of about 20°, and in a few 

 moments it will be seen that the crystalline germs have increased 

 sufficiently to show themselves through the whole mass of the 

 liquid." But it is important to note that if the tubes are exposed to 

 this high temperature " without having been previously exposed to 

 the low temperature favourable to the birth of germs, no crystallisa- 

 tion of betol shows itself, even after a long trial." 



The exact conditions under which, in liquid salol, a spontaneous 

 generation of crystals becomes possible is unknown, although it is 

 well known that crystallisation may be immediately induced in such 

 a hquid by inoculating it with microscopic crystals of this substance 

 at some temperature below 39'5° C. It is almost the same with 

 glycerine. Up to 1867 this substance was only known in the fluid 

 state, but then, after exposure to a low temperature and mechanical 

 shocks, during a long railway journey from Vienna to England, a 

 quantity of it was found to have become converted into a mass of 

 white acicular crystals. This change to the crystalhne state has 

 been observed on a very few occasions since, but the conditions 

 have not yet been accurately ascertained. It cannot be induced at 

 will, though it can always be set up at once when the liquid 

 glycerine is inoculated with a crystal of that substance ; and, as 

 with betol, the process is rendered more rapid by heat up to a very 

 moderate point. The heating must, however, only be slight, 



4 



