250 SMITH : 



a reading of the temperature of one leaf, and then transfer the 

 connecting wires of the galvanometer to the mercury cups of 

 the other apparatus and take a reading of the temperature of 

 the other leaf. 



This transference occupied only a very short space of time, 

 and it was thus possible to take readings of both leaves under 

 similar conditions. The wires could easily and quickly be 

 transferred back again, and in this way it was possible to take 

 a series of readings crossing from one leaf to the other several 

 times — a series from which a reliable average temperature 

 could be obtained. 



The two sets of apparatus were first carefully compared and 

 standardized. This was done with one junction of each in air 

 and the other in water in a room of the laboratory. It was 

 attempted to make the two sets as much alike as possible, 

 special care being taken that the very thin wires composing the 

 thermo-junctions should be of equal length. The resistance 

 of these wires is so high that any change in their length alters 

 the deflection per degree very considerably. In spite of this 

 attempt, the two sets did not register the same temperature 

 when the temperature of the air in which they were placed was 

 the same. Their deflection per degree was the same, but one 

 apparatus registered -8° C. higher than the other all through. 

 A curve was made of the galvanometer readings of both plotted 

 against the air temperatures as registered by an accurate 

 thermometer. All subsequent readings of the galvanometer 

 were referred to this curve, and thus the proper correction was 

 always applied. 



The following table gives the results for mature leaves of 

 Amherstianobilis : (a) with stomata inwards, (6) withstomata 

 outwards, obtained by using this double apparatus : — 



