160 TRANSPIRATION AND ASCENT OF SAP ch. 



smaller test-tubes to be moved simultaneously by raising 

 and lowering the upper ends of the pine supports, when 

 the upper bung is in position and the freezing-chamber 

 is closed. The double lead may be easily introduced, or 

 withdrawn, from the perforation in the upper bung by 

 means of a narrow slit opening into that perforation from 

 the side of the bung. 



From these arrangements it will be seen that the method 

 has been rendered a comparative and differential one, and 

 consequently the corrections necessary for the thermo- 

 metric methods may be partly or wholly dispensed with 

 here. Both test-tubes gain the same amount of heat from 

 the stirring. With regard to the loss of heat to the 

 freezing-bath, the water will tend to lose heat more 

 rapidly, owing to its higher temperature. This difference 

 is rendered negligible by the way in which the water 

 freezes. A continuous layer of ice always separates out 

 against the wall of the test-tube, and forms a screen 

 between the bath and the water in which the junction 

 moves. 



Again, the velocity with which the ice and the liquids 

 in the tubes come into equilibrium, depends on the 

 amount of ice present, its surface, its fineness of division, 

 and the energy of the stirring. 



In the solution less ice will separate than in the water 

 for a given temperature of the freezing-bath ; but, at 

 the same time, it is, in practice, found to be more 

 finely divided. These two differences will act in opposite 

 directions. 



The calibration-curve given in Fig. 26, which is sensibly 

 a straight line, shows that these errors practically neutralise 

 each other, and that in the working of the method the 

 galvanometer-deflection is proportional to the true depres- 

 sion of freezing-point of the solution examined. 



Reversing key. Bearing in mind the desirability 

 of eliminating all needless junctions from the circuit, 



