CHAPTER VI 



PERIODICITY 



Periodic Precipitation. — A phenomenon is said to be periodic 

 when it varies in time and space and is identically reproduced 

 at equal intervals. We are surrounded on all sides by periodic 

 phenomena; summer and winter, day and night, sleep and 

 waking, rhythm and rhyme, Hiix and reflux, the movements of 

 respiration and the beating of the heart, all are periodic. 

 Our first sorrows were appeased by the periodic rhythm of 

 the cradle, and in our later years the periodic swing of the 

 rocking-chair and the hammock still soothe the infirmities of 

 old age. 



Sound is a periodic movement of the atmosphere which 

 brings to us harmony and melody. Light consists of periodic 

 undulations of the ether which convey to us the beauty of 

 form and colour. Periodic ethereal waves waft to us the 

 wireless message through terrestrial space and the radiant 

 energy of the sun and stars. 



It is therefore not to lie wondered at that the phenomena 

 of diffusion are also periodic. According to Professor Quinke 

 of Heidelberg, the first mention of the periodic formation of 

 chemical precipitates must be attributed to Runge in 1885. 

 Since that time these precipitates have been studied by a 

 number of authors, and particularly by K. Liesegang of 

 Dlisseldorf, who in 1907 published a work on the subject, 

 entitled On Stratification by Diffusion. 



In 1901 I presented to the Congress of Ajaccio a number 

 of preparations showing concentric rings, alternately trans- 

 parent and opaque, obtained by diffusing a drop of potassium 



fei-rocvanide solution in gelatine containing a trace of ferric 



67 



