190 
Journal of Agricultural Research 
Vol. XXVI, No 4 
It will be noted, by referring to the chart taken in the compartment, 
that between the points marked the temperature was fairly constant 
for a given setting of the thermal regulator; also, that the portion to 
point io, when considered with that between io and n, shows very 
distinctly the effect of heating back with electricity. The portion be¬ 
tween io and ii shows with equal clearness the degree of control obtain¬ 
able with this equipment when the control is effected by starting and 
stopping the brine pump automatically by means of thermostat located 
in the cold chamber. 
TYPE OF REFRIGERATING MACHINES 
In the production of low temperatures for experimental work in con¬ 
nection with growing plants, the medium employed in the refrigerating 
plant should be carefully considered. In most types of refrigerating 
machines leaks of the refrigerant from the various joints in the system 
are likely to occur sooner or later and should ammonia be the refrig¬ 
erant the results would undoubtedly prove disastrous to the gijpwing 
plants. Carbon dioxid, on the other hand, would probably accelerate 
their growth to an extent that might interfere with the experiments. 
The refrigerating machinery, therefore, should be kept entirely away 
from the growing plants, unless the apparatus is of a type in which the 
refrigerating medium is hermetically sealed within the machine. In any 
low-temperature work where it is necessary or desirable to evaporate 
the refrigerating medium directly in the cold chambers, great care should 
be exercised to prevent leaks in the pipe coils. The pipe coils preferably 
should be continuous, that is, without joints. In case joints are neces¬ 
sary they should be carefully made and then soldered. 
Unless very low temperatures are desired, the brine-circulating sys¬ 
tem offers, perhaps, the best solution. With this system the brine may 
be reduced to a low temperature in an insulated tank by evaporating 
the refrigerating medium in coils which are immersed in the brine; and 
the low-temperature brine in turn may be circulated through coils lo¬ 
cated in the cold chambers. The advantages of this system are: There 
is little danger from leaks from the brine coils; the temperature in the 
cold chambers may be controlled between closer limits; it is possible to 
store up a considerable amount of refrigeration by having a large volume 
of brine which is available in case of a temporary breakdown of the 
refrigerating plant. 
The disadvantages of a brine system are: A greater initial cost, due to 
having to install both direct-expansion coils in the brine tank, and brine 
coils leading to the cold chambers and in the chambers themselves; a 
pump for circulating the brine through the coils and the additional cost 
of operating the pump; the cost of additional power due to having to 
operate the refrigerating machine at a lower back pressure in order to 
compensate for the double heat transfer from the air in the cold chambers 
to the brine and from the brine to the evaporating refrigerant. 
