52 TRANSPIRATION IN A DESERT PERENNIAL. 
and so if sufficient quantity be obtained at the beginning of the experiment 
to last for a set of readings the specific heat need be determined but once. 
Loss or gain of heat from the surrounding air by the vessels may be pre- 
vented by two methods: (1) By surrounding the vessel with a non-conducting 
_ material; (2) by cooling the liquid to a temperature the same number of 
_ degrees belowroom temperature that the final temperature will reach above 
room temperature, thus climinating the error on the theory that the heat 
lost to the air is equal to the heat gained from the air in the same length of 
Fic. 24.—Apparatus for determination of leaf-temperature. 
time. The latter requires a previous knowledge of the temperatures and 
amounts, and this is wasteful of leaf material. The former method is 
difficult to carry out in its ideal form, but by keeping the temperatures near 
air temperature, and the use of a predetermined “curve of cooling,” very 
accurate results may be obtained with well insulated vessels. 
The apparatus used was made for the purpose and is merely an adaptation 
of the ordinary calorimeter with a jacket. From fig. 24 it will be seen that 
the apparatus consists of a calorimeter suspended nearly to the bottom of a 
