LEAF TEMPERATURE. 53 



silvered Dewar vacuum beaker. The calorimeter cup is made of very thin 

 copper, nickel-plated, and polished to a bright surface on the outside. A 

 wooden cover, with holes for thermometer and stirrer, fits snugly in the top, 

 while around the outside is a wooden rim which fits closely to the sides of the 

 beaker, giving no chance for entrance and exit of air. This wooden rim is 

 suspended from the top of the Dewar beaker by means of metal wires. 

 Nowhere does the calorimeter cup touch the beaker or the metal wires. 

 Cotton is used for a cover at the top, as an extra precaution. A certified 

 one-tenth degree thermometer was used in the experiments. 



That the apparatus is well protected from exchange of heat with the sur- 

 rounding air, when temperatures within 5 degrees of the outside air are used, 

 may be seen from the fact that when the calorimeter cup was filled with 

 turpentine at 5 degrees below air temperature, then put in its position in 

 the beaker and left in the sunshine in August for 15 minutes, a rise of only 

 0.2 degree occurred. Further, the regular rate of warming or cooling, as 

 indicated by all the experiments, showed that a curve of cooling could be 

 depended upon for use in the small ranges of temperature which obtained 

 in the experiments. 



In order to find the temperature of a leaf by immersing it in turpentine, 

 the following quantities must either be known already or must be found: 

 (1) the specific heat of the turpentine used; (2) the amount of heat absorbed 

 by the calorimeter; (3) the specific heat of the leaves. 



(1) The specific heat of the turpentine was determined according to the 

 customary method followed by physicists of using a double calorimeter of 

 capacity of over 500 c.c. Three determinations were made which differed 

 in the third decimal place only, results being 0.409, 0.410, 0.411. 



(2) The amount of heat absorbed by the calorimeter system was de- 

 termined as follows. The calorimeter cup with thermometer and stirrer 

 in place was filled with a weighed amount of turpentine to a marked 

 level, this marked level being used subsequently as a guide for the level 

 of the liquid after the leaves were put in. The vessel with the turpen- 

 tine was cooled to about 12C., then weighed and put in place in the Dewar 

 beaker, where it was allowed to come to partial equilibrium. When the 

 rate of warming had slowed down to about one-tenth degree a minute a 

 weighed amount of turpentine at air temperature was quickly added, the 

 mixture filling the vessel about three-fourths full. The exact temperature 

 of each liquid was taken immediately before the mixing. The cover was 

 instantly replaced and the mixture stirred gently and continuously. The 

 time of the transfer occupied 2 to 5 seconds. The time and temperatures 

 were then read at intervals of half-minutes until equilibrium was reached 

 or until the rate of change equaled the rate of change which had been 

 previously shown to exist for the temperature obtaining. 



(3) In order to determine the specific heat of the identical leaves used each 

 time and to get them in the same condition in which they were used for the 



