184 
Journal of Agricultural Research 
Vol. XXVI, No. 4 
the refrigerating coils, and when the air is heated back a low relative 
humidity will result. In many branches of work a low relative humidity 
is undesirable for the reason that it tends to desiccate the specimens. 
In order to lower the temperature inside a chamber it is necessary to 
remove the heat. The quantity of heat to be removed from a chamber 
depends upon the difference in temperature between the inside and the 
outside of the room, upon the rate at which the heat passes through the 
surfaces of the room, and upon the quantity of heat produced or absorbed 
by the contents of the room. It is obvious, therefore, that in order to 
reduce the quantity of heat that must be removed, and hence the work 
required to remove it, adequate insulation should be provided. The 
better the insulation the more perfect the control of temperature, for the 
reason that the change in temperature is not so rapid when the refrigera¬ 
tion is discontinued, and therefore the quantity of heat that must be 
removed in order to establish constant temperature conditions is dimin¬ 
ished. Furthermore, adequate insulation tends to maintain a constant 
temperature, and the rise in temperature, with the refrigeration entirely 
cut off, is slow, so that it becomes possible in some cases for the refriger¬ 
ating plant to be shut down for several hours with only a few degrees' 
rise in temperature. This is important for the reason that, should the 
temperature control equipment break down for any reason, some time 
could be allowed for repairs, under average conditions, with a temperature 
rise that would not materially affect the experiments. 
The experimental plant, as originally constructed, was to be used for 
maintaining the temperature of the culture room at 18 0 C. (64.4° F.). 
This part of the control was employed in work on the absorption of min¬ 
eral nutrients by crop plants, using the conductivity method for deter¬ 
mining the daily salt .concentration in water cultures. A temperature 
accurately controlled to ±o.i° C. was convenient for the reason that it 
did away with temperature corrections in calculating the salt concen¬ 
trations, and also made results in consecutive series comparable. With 
this equipment it was possible to maintain a temperature of 18 0 C. 
±o.i° for long periods of time. In fact, interruptions were caused only 
by the discontinuance of the electric power, or breaks in the control 
system, and not because the apparatus itself failed to function. 
The original installation, however, has now been extended for pur¬ 
poses to be mentioned later. The plant is divided into two parts, one 
for indoor and one for greenhouse work, each of which will be consid¬ 
ered separately. 
INDOOR COMPARTMENTS 
The arrangement and construction of the indoor plant is shown in 
figure 1. The walls, floors, and ceilings are well insulated with cork 
board, and finished inside with a half-inch coat of hydraulic-cement 
plaster, so that the rooms can be washed out thoroughly and disinfected 
when necessary. There are three rooms in all—namely, a coil room, 
a culture room, and an instrument room, as marked on the plan. 
The coil room is intended primarily as a “reservoir of refrigeration” 
from which cold air is drawn to maintain a constant temperature in the 
other two rooms. The temperature in the coil room is thermostatically 
controlled, the thermostat operating to stop and start the refrigerating 
machine on a temperature variation of about 3 0 F. 
The temperature in the culture room and in the instrument room is 
maintained practically constant by drawing cold air from the coil room 
