492 



Mr. B. V. Hill on Accidental 



times, gelatinization takes place more rapidly than before. 

 A fresh solution containing 1 gram to the litre gave a retar- 

 dation of -000125 \, which was independent of the speed. 

 A solution of similar concentration which had been made by- 

 diluting a stronger one, and which had been boiled and 

 cooled several times, gave a retardation of from '00021 \ to 

 •00042 A,, decreasing with increase of speed. After again 

 diluting with an equal volume of water, it was boiled and 

 cooled, and then gave a retardation of from '00025X to 

 •00040 X, decreasing with increase of speed. _ 



The behaviour of solutions not prepared in the usual way 

 sometimes appears anomalous. A solution containing 10 grains 

 to the litre was diluted to contain 7£ grains, and examined 

 without boiling. The results are given in Table V. 



Table V. 



Temp. 20°. 

 Zero. 



Speed 41. 



Setting of 

 analyser. 



Kotation. 



AX X 10*. 



10-10 



10-08 



9-87 



9-79 



9-78 



10-14 

 10-13 



9-93 



9-84 



9-83 



Means 



6-04 

 •05 

 •06 

 •05 

 ■05 



224 

 2-80 

 336 

 2-80 

 2-80 



•05 



2-80 





Here the rotation of the plane of polarization is constant, 

 but the zero of the analyser has moved 0°'32 to the left. 



Since the increase of double refraction reaches a limit with 

 change of speed, it seemed possible that this maximum con- 

 dition might occur with static strains. Parallelepipeds of 

 jelly were accordingly moulded in small glass cells. They 

 were then removed and sheared by displacing the upper 

 surface. A jelly made by adding 20 grams of gelatine to 

 100 c.c. of water, forming a solid of considerable rigidity, 

 gave, under these conditions, a rotation of the analyser of 

 50°, representing a relative retardation of '28 A.. As long as 

 the stress was applied to the block the double refraction 

 remained constant, and no tendency to slip back appeared. 

 It was now endeavoured to find how small a percentage of 

 gelatine could be used and still obtain the static strain. When 

 the jelly was too weak to bear its own weight, the edges of 



