Double Refraction in Liquids, 49i 



The double refraction in a liquid is less at high than at low 

 temperatures. In this, one must consider the temperature 

 to which tne solution has been cooled, and not that at which 

 the observation may be made. A solution containing four 

 grams' to the litre gave, at 27°, a relative retardation of 

 •00021 X. The same solution at 17° gave -00039 X. If it be 

 now heated to 27° the effect is not -00021 A, but -00030 \. 



The effect of the temperature upon the depolarization is of 

 the same nature as its effect upon the double refraction. This 

 dispersion is strong - even in dilute solutions. A solution con- 

 taining 3 grams to the litre was allowed to stand 24 hours 

 at 15°. The least motion of the cylinder caused the whole 

 field to brighten, and the rotation of the analyser produced 

 no minimum point. The solution was heated, and the cylinder 

 rotated slowly. The depolarization decreased very slightly to 

 the temperature 33°, when it disappeared quite abruptly. No 

 effect of double refraction could be seen at this point. When 

 a similar solution was cooled to 33°, a retardation of -000098A- 

 was observed with a speed of 24 revolutions per second. The 

 depolarization and double refraction disappeared together at 

 the same temperature in solutions containing 5 and 10 grams 

 to the litre. This is the melting-point of solid jelly. Gelatine, 

 if put into water, will swell and absorb water, but will not go 

 into solution until this point is reached. 



The coudition of a gelatine solution depends upon the time it 

 has been allowed to stand. Frass * found that the elasticity of 

 jelly nearly doubles in twenty-four hours ; but after that there 

 was little change. In this weak solution the whole effect is 

 so small that considerable time must pass before trustworthy 

 measurements can be made. A fresh solution containing 

 3 grams to the litre gave a retardation of "000 16 \ at 30°. 

 A similar solution allowed to stand four hours at 30° without 

 stirring gave a retardation of '000224 X. After standing 

 twenty-four hours at 25° there was a retardation of "000355 \, 

 When a solution containing 3 grams was allowed to stand 

 seventy-five hours at 22°, the depolarization became so strong 

 as to prevent measurements. When the temperature of the 

 solution remained a little above 33° no change took place. 



The manner in which a solution has been made has much 

 to do with its behaviour. For example, 200 grams of gelatine 

 was dissolved in 3 litres of water, and this was then diluted 

 to 10 litres. When examined, the depolarization prevented 

 observation. It was immediately boiled and cooled to the 

 temperature of the first experiment, 27°; measurements could 

 now be made. If a solution is heated and cooled several 

 * Frass, Wied. Ann. vol. liii. p. 1074. 



