Periodical phenomena. 1 1 7 



started into combustion by a flame, burns slowly 

 away ; but when started by a detonating substance, 

 detonates violently. Guthrie has also discovered that 

 if a melted cryohydrate (e. g. a chilled saturated 

 aqueous solution of a solid salt) is cooled to a certain 

 greater extent, it will not solidify nothing separates 

 out, although the solution is four or five degrees below 

 its proper solidifying point. If a little crystal of ice 

 be then thrown into it, nothing separates but ice, 

 which comes to the surface. If we throw in a little 

 anhydrous salt, nothing but the anhydrous salt se- 

 parates out, and that sinks to the bottom. But if 

 we throw into it a crystal of a previous crop of 

 cryohydrate, then nothing but the cryohydrate se- 

 parates. In this case also, like evidently excites like 

 only, in obedience to physical laws. (Addresses, 

 Science Conferences ; South Kensington Museum, 

 1876; Vol. 2, p. 108). Even two clocks, when hung 

 near each other, against a board or surface which 

 readily transmits vibrations, have been known to ex- 

 hibit, by synchronous action, an apparent sympathy, 

 which changed their rate of going. 



Periodical phenomena, also, brought about by defi- 

 nite causes, occur in mental, as well as in physical 

 phenomena. In the former we have the phenomena 

 of sleep, and in the latter, definite causes produce 

 summer and winter, day and night, the tides, cycles of 

 solar spots, maxima and minima of magnetic inten- 

 sity, etc., etc. 



Conversely to the manifestation of the principles of 



