[ 49 j 



IV. 



A THEUMO -ELECTRIC METHOD OF CRYOSCOPT. 



By henry H. DIXON, Sc.D., F.R.S., 



University Professor of Botany in Trinity College, Dublin. 



[Read January 24. Ordered for Publication February 14. Publislied April 20, 1911.] 



In some work' on the determination of the osmotic pressures of plant-juices 

 I have found a thermo-electric method of measuring the freezing-points of 

 small quantities of solutions so convenient and capable of such unexpected 

 accuracy that I venture to think a detailed description of the method may 

 prove of some use. 



On first thoughts it seems surprising that thermo-couples have not been 

 in general use for determining the freezing-points of solutions. In the first 

 place, it is evidently possible to make the thermo-electric method a difEerential 

 one, viz. J comparative 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° C. With this sensitiveness 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 bulkiness and fragility 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 

 thermo-couples have been used in ciyoscopy, 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 galvanometer is used to give a good deflection 

 for small temperature-difierences, it is found also to be deflected by 

 temperature-differences acting on accidental junctions in the circuit. These 



1 Henry H. Dixon and W. U. Gelaton Atkins, On Osmotic Pressures in Plants ; and on a 

 Thermo-Electric Metliod of Determining Freezing-Points. Proe. Koy, Dub. Soc, vol. xii.. No. xxv., 



p. 275. 



SCIENT. PKOC. B.D.S. , VOL. XUI., NO. IV. 



