334 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
When comparisons are made at temperatures l)elow freezino-, the low-temperature 
comparator has been found to answer all requirements in a most admirable manner, 
and has been used to lu-oduce temperatures as low as 65° F. below zero. Still lower 
temperatures, it seems, are easily obtained, though up to this time no occasion to go 
to lower temperatures 1ms arisen. 
The amiular iron JiasJc A is made of the best iron boiler-ilue tubing, strongly fitted 
with the headpieces h h'. The outside diaineter of the two tubes is, respectively, (i 
and 9 inches, the tlask beiiig 9 inches high and containing about 1 gallon. The whole 
stands within a copper can, itself placed within a larger wooden jacket, the interspace 
being filled Avith cotton. A top plate of metal, c c, partly covers the can, to which it 
is tightly screwed, leaving a circular opening at the center of about the same diameter 
as the inside of the llask A, Tlie whole is a.gain covered, except the opening, by a 
loosely fitting Avooden plate, w tv. The alcohol with Avhich the can is filled can be 
CxTT .50. — LoAv-teiu])erature com])arator. 
stirred in a most thorough manner by means x)f a disk dasher, S, moving near the 
bottom of the can and within the iron llask. This passes the alcohol in a rapid 
manner from the inner portion to the outside around the iron flask and vice versa, Avitli 
the most satisfactory results. 
The iron flask, by means of the sere av- threaded outlet of its valve, can be joined 
by a very short piece of pipe to a flask of ammonia, Avhich, in this case, must always 
be upside down in order to run off the liquid. If the flask A contains air, generally 
only a small quantity of li(piid ammonia Avill enter, and this is best effected by opening 
the valves quite promixtly. To further charge the flask it is first necessary to drive 
out the air, for which the stock ammonia flask is disconnected and a rubber tube or 
other outlet attached. The ammonia in the gaseous state, Avhen permitted to escape 
from A, carries with it tlie air also. The temperature of the alcohol is gradually 
loAvered, and if an additional supply of ammonia is needed any quantity may be 
drawn from the supply flask out of winch the ammonia Avill now be strongly forced by 
its greater vapor pressure, due to the difference of temperature of the two flasks. 
The ammonia gas is best disposed of by passing it into a bottle or other vessel of 
water, which is thus, in time, converted into excellent aciua ammonia. 
