278 Scientific Procccilings, Ro/jul Dublin Society. 



sioii of the freeziug-point of a solution and its osmotic pressure are directly 

 dependent on its concentration.' 



Beckmann's apparatus, wbicli is usually employed in these determina- 

 tions, involves the use of a thermometer with a large bulb which m\ist be 

 immersed iu the solution whose freezing-point is to be determined. The 

 size of the bulb necessitates the use of a considerable quantity of the solution, 

 viz., 12 to 15 c.c. as a minimum. Such large requirements seemed to us to 

 preclude the application of the method to the determination of the osmotic 

 pressures of the sap of transpiring organs, of which but small quantities 

 can be conveniently obtained. Tiiis objection applies all the more strongly 

 to the other more elaborate methods of determining the freezing-points of 

 solutions as described in Hamburger's Osmotischer Dnick und lonenlehre. 



In order to circumvent this difficulty, it occurred to us to replace the 

 mercurial thermometer by a thermocouple, and to compare directly the 

 freeziug-point of water with tliat of the solution. Past experience^ had shown 

 that with a suitable galvanometer and a single element of nickel and copper, 

 it is easy to obtain a motion of the spot of light on tlie galvanometer scale 

 of one millimetre for a difference in temperature at the junctions of 0"01° 0. 

 As the variations in the freezing-point of tlie sap of various plants and under 

 various conditions are large, this is ample delicacy. 



Appavaius. 

 The couple for observing the temperature was made of a piece of silk- 

 insulated nickel wire 0'15 mm. in diameter, and 30 cm. long. Each end of 

 this was stripped for a few millimetres, and soldered to a fine insulated 

 copper lead. The nickel wire was bent in a V-form; and each lead was twisted 

 on the arm of the V to which it was attached. From the angle of the V the 

 two leads were twisted together till they again diverged from one another to 

 make connexion with the terminals of the galvanometer. Each arm of 

 the V was bound on a rigid rod to give it the necessary stiffness. This 

 support was either a piece of drawu pinewood injected with paraffin or 

 a goose-quill. The silk-insulation of the wires and the junctions was 

 smeared over with several coats of rubber-solution to water-proof it and 



1 H. J. Hamburger, Osmotischer Druck und lonenlehre. Bd. i. "Wieshaden, J. V. Bergraann. 

 1902. 



[While this paper was in press we received through the kindness of Professor F. Cavara two 

 papers containing important results obtained by tliis method, viz. F. Nicolosi-Eoncati, Ricerche 

 su la Conduttivita Elettrica e la Pressione Osmotica nei Vegetali. Bull, dell' Orto Botanico della. 

 II. Univ. di Napoli, T. ii., Fasc. 2°, 1907 ; and G. Trinehieri, Su le Variazioni della Pressione 

 Osmotica negli Organi della Salpichroa rhomboidea, Miers., ibid., T. ii., Fasc. 4°, 1909.] 



- Henry H. Dixon, Observations on the Temperature of the Subterranean Organs of Plants, Trans, 

 lluy. Irish Acud., vol. xxxii., sect. B., part iii., p. 145 el scq. 



