156 TRANSPIRATION AND ASCENT OF SAP ch. 



can be conveniently obtained. This objection applies all 

 the more strongly to the other more elaborate methods 

 of determining the freezing-points of solutions. 



Use of thermocouples. In order to circumvent 

 this difficulty, it was decided to replace the mercurial 

 thermometer by a thermocouple, and to compare directly 

 the freezing-point of water with that of the solution. 



It seems surprising that thermocouples have not been 

 in general use for determining the freezing-points of solu- 

 tions. In the first place, it is evidently possible to make 

 the thermo-electric method a differential one, viz., com- 

 parative of the freezing-point of the solution to be examined 

 with that of pure water under the same conditions, and so 

 it would seem most of the corrections necessary in the 

 thermometric method would be rendered needless. Thermo- 

 couples have great possibilities of sensitiveness, e.g., it is 

 by no means difficult to obtain by their use a deflection 

 of the light-spot on a galvanometer scale amounting to 

 1 mm. for a difference of 0*0001 . With this sensitive- 

 ness they can be made with a very small heat-capacity, 

 and will consequently take up the temperature of their 

 surroundings quickly. Their minute size and ease of 

 manipulation contrast very favourably with the bulki- 

 ness of the ordinary freezing-point thermometers. Absence 

 of parallax in reading the scale and the ease with which 

 couples having various ranges may be constructed will 

 also occur as advantages. 



Notwithstanding these attractions, I have not been able 

 to find that thermocouples had been used previously in 

 cryoscopy, although in many researches their properties 

 would be of value. This is probably to be explained by 

 the erratic behaviour they exhibit when set up without 

 special precautions. When a sufficiently sensitive galvano- 

 meter is used to give a good deflection for small temperature- 

 differences, it is found also to be deflected by temperature- 

 differences acting on accidental junctions in the circuit. 



