120 MISSOURI BOTANICAL GARDEN. 
lege, Dublin 2: 47-83. 1910.) in which they describe the 
preparation and use of the apparatus used in these experi- 
ments. A photograph of the complete apparatus used 
appears on the accompanying plate. It consists, in the 
main, of a thermo-couple made up of nickel wire, the ends 
of which are soldered to copper leads which connect with 
the terminals of the galvanometer. The junctions are fitted 
into two small glass tubes into one of which the solution 
to be frozen is placed (2-3cc.), and in the other, distilled 
water. The tubes are mounted in such a way that they 
can be inserted into a convenient vessel, which may be sur- 
rounded by the freezing mixture. The couple itself is sup- 
ported by wooden rods so as to allow of constant stir- 
ring during the operation, and for removal and cleaning 
of both the junctions and the tubes. <A reversing key, 
consisting of an ordinary spring clip, containing the leads 
from the galvanometer on its inner jaws, and the leads 
from the couple on the surface of a glass plate, is in- 
serted in the circuit, and the whole submerged in coal 
oil to eliminate the effect of possible differences of tempera- 
ture of the terminals of the galvanometer. A diagram of 
the apparatus is shown in the paper already referred to. 
The only essential difference in the apparatus as used, 
occurs in the use of German silver wire in the place of 
nickel in the thermo-couple, and in the use of a Leeds and 
Northrup Co. (Philadelphia) galvanometer, instead of the 
Ayrton-Mather instrument used by Dixon and Atkins. In 
using the apparatus it is essential that the whole of the 
couple be at the temperature of ice, for if any part of it 
remains out of the freezing temperature it acts itself as a 
couple and results in an inaccurate reading. In practice, 
after the distilled water has been placed in one of the tubes 
and the sap to be tested in the other, the holder contain- 
ing the tubes and the couple is placed in a freezing mix- 
ture of salt-water and ice. The mixture surrounding the 
potato tube of the illustration may be used for this pur- 
pose. In a few moments the ice crystals may be seen 
