DETERMINATION OF THE FREEZING POINT 53 



gained in a \'ery heavy series of determinations. That they are 

 thoroughly practical is perhaps evident from the fact that by 

 tlieir use we have been able (beginning with the tubes in the 

 freezing mixture) to carry out the determination of specific 

 gravity (by weighing in a pycnometer), total solids, freezing point 

 depression and electrical conductance of more than 800 samples^ 

 of sap in about two months time. Three of us took part in this 

 work, but two could only give half of their time to this phase. 



The rapidity with which large series of determinations can be 

 carried out is our justification for the detail in which these minor 

 points of technique are described. In the more exact sciences of 

 physics and chemistry where external conditions can be closely 

 controlled the number of individual experiments which are 

 necessary is generally not large. But in biology, where there are 

 so many unknowable factors which have their influence upon 

 one's samples the value of any piece of work is much more nearly 

 proportional to the number of observations upon which it is 

 based. 



^ Two readings were made when there was the slightest suspicion of error, 

 but not otherwise. The Wheatstone bridge and the conductivity cell were fre- 

 quently checked against 0.10 N KCl. and the thermometer tested by the freezing 

 of conductivity water. 



