1 68 TRANSPIRATION AND ASCENT OF SAP ch. 



position ; and the test-tube should again be touched. 

 When it is certain that almost all the ice is melted in the 

 solution, it is left in the freezing-chamber and allowed 

 to cool. Meanwhile connection is broken by removing the 

 clip from the support in the petroleum key. 



When it is judged that radiation has cooled the solution 

 nearly to its freezing-point, connection is again made by 

 the clip, and stirring is recommenced. The spot of light 

 then travels to near its previous resting-place, or possibly be- 

 yond it. Supercooling may proceed, and the spot of light 

 will slowly travel indefinitely beyond its previous position, 

 or crystallisation may supervene, and the spot will return 

 somewhat on its path and tend to take up a steady position. 

 In the latter case connection is broken at the clip, and 

 the mirror allowed to swing free. Connection is again 

 made, and, after 75 sees., during which vigorous stirring 

 is kept up, a reading is made. The current is then reversed, 

 and at the end of 75 sees., another reading is made on 

 the other side of the zero point. If, however, super- 

 cooling proceeds, and crystallisation does not automatic- 

 ally occur, it is necessary to inoculate the solution with 

 a little hoar-frost. The inoculation should be carried out 

 when the spot of light has definitely passed the limit of 

 the first deflection. If it is allowed to cool too far, 

 much ice will separate, and the concentration of the 

 solution left over will cause too large a depression ; if, 

 on the other hand, it is inoculated just at its freezing- 

 point, so little ice separates that the solution in parts 

 may continue for some time supercooled, and we may 

 get too great a deflection. Experience shows that the 

 smallest depression is obtained if the solution is allowed 

 to cool 0T to 0*2 below its freezing-point before in- 

 oculation. 



It will often be found that the mean of the second pair 

 of readings indicates a larger deflection than that of 

 the first pair by about 1 per cent. This seems to be 



