286 Scientific Proceedings^ Royal Dublin Society. 



E.r2yerimcnfnl. 



Having set up the galvanometer and scale, and couuected the leads of 

 the spring-clip to the terminals of the galvanometer, freshly boiled distilled 

 water is introduced into one of the test-tubes in the frame, and about 3 c.c. of 

 the pressed sap into the other, and the junctions of the thermo-couple are 

 immersed in them. The freezing-jacket, consisting of ice and brine, is 

 adjusted to a temperature 0'5°- 1'0° C. below tlie suspected freezing-point of 

 tlie sap. The two test-tubes are cooled in a freezing-mixture, and a lining of 

 ice is formed in that one vrhich contains the distilled water. The junction 

 stands in tlie liquid surrounded by a shell of ice. The sap is by this time 

 probably cooled below its freezing-point, but will only begin to freeze after 

 its "inoculation" with an ice-crystal or the local application of intense 

 cold. When by either of these means crystallization has been started, the 

 frame carrying the test-tubes is immersed in the freezing-chamber ; and 

 connexion is made between tlie couple and galvanometer by setting the clip 

 on the terminals of the former. During this time tlie junctions are kept 

 stirred about in their test-tubes by moving the clamp liolding the supports. 

 Immediately on connexion being made, the spot of light moves from zero, and 

 assumes a position due to the depression of the freezing-point of the sap 

 below 0°C. The first observation made in this way usually gives too 

 great a depression, because the sap has probably been supercooled ; and 

 consequently a great deal of ice separates out, leaving the remaining liquid 

 much concentrated. This ice should now be very gradually melted. When 

 the test-tubes are again introduced into the cooling-cli amber, after a very 

 small amount of supercooling, crystallization usually supervenes, as minute 

 ice-crystals remain over after the first freezing, unless the sap has been warmed 

 considerably above 0° 0. This leads to a slight rise in temperature, which is 

 recorded by a small movement of the spot of light towards the zero. Wlien 

 the spot of light has come to rest — in the meantime stirring is continued — 

 a reading is made, and immediately by means of the clip the current is 

 reversed. The spot of liglit now travels to the other side of the zero ; 

 and when it is again motionless, its position on the scale is recorded. The 

 mean of the two deflections (by referring to the deflection curves and Raoult's 

 temperature curve given in fig. 2) afl'ords the measure of the depression of 

 the freezing-point. After some practice and experience reliable readings are 

 obtained with certainty. 



In the following tabular statement of our results, the depression of the 

 freezing-point of the sap is found in the column under A. Tlie osmotic 

 pressure given under P has been calculated from the known relation between 



