172 TRANSPIRATION AND ASCENT OF SAP ch. 



From these figures it appeared that when the freezing- 

 bath is less than T2 below the freezing-point of the 

 solution under examination, a small alteration in the 

 temperature produces a greater effect on the deflection 

 than when the bath is about T5 below the solution. It 

 consequently seemed best to adjust the freezing-bath 

 about 1 *5 below the suspected temperature of the freezing- 

 point. 



As has already been pointed out, the influence of the 

 temperature of the freezing-bath on the apparent freezing- 

 point deflection is due to the difference in the behaviour 

 of water and salt solutions on freezing. In the latter 

 the crystals remain separate and the ice is finely divided. 

 The difference of density between it and the solution 

 causes it to rise up somewhat more rapidly, tends to aggre- 

 gate it at the upper surface, and so permits the lower 

 layers of the solution to supercool to a small extent. In 

 the distilled water, on the other hand, the ice adheres to 

 the walls of the tube, and forms a lining to it, so that 

 supercooling of the lower layers is less favoured. 



The convergence temperature and the velocity of heat 

 exchange between the freezing-bath and the contents of 

 the tubes are dependent largely on the heat-capacity of 

 the solution, and consequently it is of importance that 

 the two test-tubes should contain approximately the 

 same amount of liquid ; otherwise the rate of exchange 

 of heat between the solution and the freezing-bath and 

 the water and the freezing-bath will be different. Thus 

 the deflection due to the depression of freezing-point 

 of a solution containing 15 gm. sodium chloride in 

 100 gm. of water was found to be 115 6 mm., when the 



