2-34 Scientific Proceedings, Royal Dublin Society. 



100 grams of water. The curve iu the figure marked / is that traced by 

 the deflections of the galvanometer in connexion with couple No. 1, 

 corresponding to the freezing-points of the same solutions. As before, the 

 ordinates represent the concentration of the solution. But the abscissse for 

 this curve represent the deflection of the spot of light in millimetres. The 

 two curves lie close to one another, because each scale-division, viz. 1 mm., 

 nearly corresponds to a deflection due to a difference of temperature of the 

 junctions of 0'01°O. Curve //is the plotting of the readings given by a 

 similar couple constructed as like the first as possible. Curve /// gives the 

 curve of couple No. 3, determined by means of the improved apparatus in 

 which the couple is completely immersed in the cold chamber ; and the zero- 

 error has been eliminated by the introduction of the petroleum reversing 

 key. It will be seen that the accuracy of the individual observations is 

 considerably increased, so that all the readings lie on a uniform curve. 

 The general agreement of the tiiree calibrations must be regarded as 

 satisfactory. 



ExlraetioK of the Sap. 



At tlie beginning it was feared that to obtain even 2'5 c.e. of sap from 

 leaves of most plants would be a matter of trouble and difficulty. Our first 

 method was to break up a weighed quantity of leaves to a nearly uniform 

 pulp in a mortar, and then to add a measured quantity of water to the pulp. 

 This water and pulp were intimately mixed together, and then as much of 

 the fluid as possible was pressed from the pulp and used for the determination 

 of the freezing-point. A separate observation gave the percentage of water 

 in the uninjured leaves. Knowing this, and the weight of the pulp and added 

 water, the dilution of the solutions caused by the addition of water to the 

 pulp was calculated ; and by suitable allowance the freezing-point of the 

 more concentrated original sap was deduced. The dilution does not introduce 

 more than a slight error. Our first seven observations were made by this 

 method of extraction. They are recorded in Nos. 1-7 of the Table of 

 Eesults. 



Later on, to our surprise, we found that the pressing of the sap from the 

 leaves was quite an easy matter. To effect it, a few leaves, their midribs being 

 removed, are taken, and crumpled up into a tight pellet. The pellet is 

 wrapped in a double coat of fine linen and placed between the jaws of a 

 powerful vice. It was found convenient to have two stout silver discs, which 

 were not appreciably attacked by the sap, between which the pellet was 

 introduced into the vice. There were marked differences in the behaviour of 

 leaves under pressure. Some gave up their juice readily when first the jaws 



