62 BURTON EDWARD LIVINGSTON 



tact with the inner surface of the cup wall. The lower end of the 

 cup is closed with a rubber stopper bearing glass or other suitable 

 tubing connection to the reservoir. This tube is also full of water. 

 The reservoir may be any suitable container; it is frequently a 

 small mouth glass bottle with cork stopper and with a file mark on 

 the neck to serve as zero point. In operation, this reservoir is filled 

 to the file mark, the instrument is allowed to operate for a time 

 period, and the reservoir is again filled, account being taken of 

 the amount of water required, which is of course the amount lost 

 by evaporation during the period. Where small readings are to 

 be taken, the tube is often connected to the lower end of a bu- 

 rette which is filled from time to time, the decrease in the volume 

 of water in the burette reservoir being read directly on the 

 scale. In all cases the level of the water in the reservoir must 

 be lower than the base of the porous surface. 



It is obvious that the porous clay evaporating surface may 

 have any desired form or size and that there is no danger of the 

 drying of a part of the surface through excessive evaporation in- 

 tensity, as is the case with the saturated paper surfaces mentioned 

 above. It is also clear that, unlike the paper surfaces, the porous 

 clay surface is in close proximity to a considerable mass of water, 

 but the magnitude of this mass, in relation to the form and 

 extent of the surface, may be modified to suit the requirements 

 of the study in hand. With the porous cup form of instru- 

 ment the evaporating surface may readily be placed at a con- 

 siderable distance from the water reservoir. 



The details of the operation of porous cups will receive atten- 

 tion farther on, after some important points regarding porous 

 surfaces in general have been considered. 



THE POROUS EVAPORATING SURFACE IN GENERAL 

 Nature of the Porous Surface 



Besides paper and porous porcelain several other solid ma- 

 terials have been employed for evaporating surfaces. Woven 

 fabric has sometimes been used. Perhaps the most frequent 

 mention of porous surfaces in atmometry, however, refers to one 



