RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 143 



Soil cans — Continued. 



can, making it possible to keep can and cover together while the 

 can is open — 



Number 1 2 3 



Diameter, inches 2 2\ 3£ 



Height, inches f If 2 



Each $0. 11 $0. 15 $0. 20 



With can and cover numbered. (In ordering, state what numbers 

 are desired.) 



No. 9184 A, each $0. 16 $0. 20 $0. 25 . 



Aluminum soil cans, 2\ by 2\ inches, with screw tops — 



Lightweight, per hundred $38. 00 



Heavyweight, per hundred 45. 00 



Cans, galvanized, 4 by h\ inches, for capillarity, moisture equivalent, 



etc. (Any sheet-metal works.) .40 



Drying ovens: 



About 

 Hot-water bath, in various dimensions from 9 inches up - $50. 00 



and up. 

 Electric, Freas, type R No. 108, inside dimensions 16 by 14 by 16 inches, 



thermostat, thermometer, etc $210. 00 



Hearson low temperature incubators, gas and electric heated, various 



sizes 140 to 360. 00 



Potentiometers and other electrical resistance apparatus. 

 Water-retention cup, for determining the maximum water retained by soil, of 

 brass 2 inches in diameter by f inch high, with diaphragm of perforated 



metal fastened about ^ inch below top, No. 9295 0. 20 



Capillary moisture pans: 



Hilgard's small circular metal pans, about 1 centimeter high and 4£ 

 inches in diameter, with perforated bottoms for determining ' 'capillary 



moisture' ' of soil, each 1. 25 



Per ten 10. 00 



Balances, glassware, reagents, etc. (Obtainable from all dealers in laboratory 

 supplies.) 



ATMOSPHERIC HUMIDITY. 



The humidity of the atmosphere is directly reflected in any such 

 water-containing object as the leaf of a plant, in which there is a 

 constant tendency to come into vapor-pressure equilibrium with the 

 atmosphere, usually through evaporation but in rare circumstances 

 through absorption. The point of equilibrium between the leaf and 

 the atmosphere will be better understood by considering the discus- 

 sion which has preceded, in reference to osmotic pressures. 



Although this constant tendency is nearly always causing the loss 

 of water from plants, the humidity of the atmosphere alone can not 

 be taken as a measure of the " evaporation stress," or rate of evapo- 

 ration, depending on the wind movement which aids in diffusion of 

 vapor, and the heat supply, principally from sunlight. For this 

 reason, when a direct measure of the evaporation stress is possible 

 through the use of some form of atmometer, ecological studies will 

 not require the measurement of atmospheric humidity except in a 



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