SAP DENSITY AND FREEZING POINTS OF LEAVES. 121 
forming in the tube containing the distilled water, and 
shortly after in the tube containing the sap. During this 
time the liquid in the tubes has been kept stirred by 
means of the rods holding the thermo-couple. The insu- 
lation on the wires of the couple should be examined 
frequently during the operation so as to remedy any defects 
caused by the rubbing of the wires on the sides of the 
tubes in stirring. As soon as crystallization has set in, 
the holder containing the couple and tubes is put into the 
potato tube, which all this time has been surrounded by 
the freezing mixture. The two terminals of the reversing 
key are connected and the latter inserted in the vessel of 
coal oil which is kept stirred. When the movements of 
the mirror of the galvanometer have ceased, the current 
is reversed, and the reading taken from the other side 
of the zero. It is well to again reverse and verify the first 
reading, since if either of the liquids in the tubes should 
be unfrozen the deflection will increase; or when the liquids 
are frozen solid a sudden falling off toward the zero will 
be noticed. The movement of the mirror after the leads 
have been connected should be steady and then come to 
a complete stop. Any deviation from this action in the 
way of erratic movements may be taken as an indication 
of poor insulation, or over- or under-freezing. A little 
practice will easily make the operator acquainted with the 
instrument, and once familiar with it, there will be no 
difficulty at all in obtaining accurate readings. The deflec- 
tions of the galvanometer used were not very great, ranging 
from two to nine millimeters. The tenths of a millimeter 
were estimated, and, consequently, many of the freezing 
points given in the tables as identical, may, as a matter 
of fact, be slightly different. A more sensitive galvano- 
meter is to be desired for accurate and definite results, but 
for the purpose in view where only the comparison is of 
importance, the instrument used is quite sufficiently accu- 
rate. It has been found that on the jar containing the 
freezing mixture, ice or frost will usually form, When the 
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