1 
2 3 
2 
2* 3J 
£ 
If 2 
. .$0.11 
$0. 15 $0. 20 
(In ordt 
sring, state what 
RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 143 
Soil cans — Continued. 
making it possible to keep can and cover together while the can 
is open — 
Number 
Diameter, inches 
Height, inches 
Each 
With can and cover numbered, 
numbers are desired.) 
No. 9184 A, each $0.16 $0.20 . $0.25 
Aluminum soil cans, 24_ by 2\ inches, with screw tops — 
Lightweight, per hundred $38. 00 
Heavyweight, per hundred 45.00 
Cans, galvanized, 4 by 5£ inches, for capillarity, moisture equiva- 
lent, etc. (Any sheet-metal works.) .40 
Drying ovens : 
| About 
Hot-water bath, in various dimensions from 9 inches up i $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, vari- 
ous 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 | inch high, with diaphragm of 
perforated metal fastened about i& 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 w T ill 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 rea- 
son, 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 ex- 
cept in a somewhat perfunctory manner, as a means of detecting 
