PERIODICITY 7 5 



of calcium nitrate. We spread the gelatine evenly over 

 a plain glass lantern slide and allow it to set. After it is 

 set, but before it dries, we place in the centre of the slide a 

 drop of concentrated solution containing two parts of sodium 

 carbonate (N T a 2 C0 3 ) to one of dibasic sodium phosphate 

 (Na,HP0 4 ). Tribasic sodium phosphate alone without the 

 addition of the carbonate will also give good results. If the 

 phosphate solution is placed on the gelatine in the form of a 

 drop, we obtain circular periodic precipitates. If it is desired 

 to make a rectilineal grating, we deposit the phosphate solution 

 on the gelatine in a straight line by means of two parallel 

 glass plates. In this way we may obtain lines of periodic 

 precipitation to the number of 500 to 1000 per millimetre, 

 forming gratings which produce most beautiful spectra. 



Pearls and mother-of-pearl both owe their iridescence to a 

 similar ruled structure, which is developed in the living tissue 

 of a mollusc. They are, in fact, periodic precipitates of phos- 

 phate and carbonate of lime deposited in the colloidal organic 

 substance of the mollusc. They have the same structure and 

 the same chemical composition ; they have the same physical 

 properties, the glow, the fire, and the brilliancy of our spectro- 

 scopic gratings. In these experiments, indeed, we have realized 

 the synthesis of the pearl, not only a chemical synthesis, but 

 the synthesis of its structure and organism. 



We have been able to make these periodic precipitates by 

 the reaction of a great number of chemical substances, giving 

 a bewildering variety of form and structure. Some of these 

 recall the form of various organisms, and especially of insects, 

 as may be seen in Fig. 18. 



All the phenomena of life are periodic. The movement of 

 heart and lungs, sleep and waking, all nervous phenomena, have 

 a regular periodicity. It is possible that the study of these 

 purely physical phenomena of periodic precipitation may give 

 us the kev to the causation of rhythm and periodicity in living 



beings 



Besides this periodic precipitation there appear to be other 

 chemical reactions which are periodic. Professor Bredig of 

 Heidelberg has lately described a curious phenomenon, the 



