DETERMINATION OF THE FREEZING POINT 51 



into the desert by automobile oi- for a sev^eral hours trip down 

 the mountains by pack mule. One of the criticisms which has 

 been urged against some of the field work on osmotic pressure in 

 j)lants is that the sample could not be worked with until profound 

 changes through evaporation or otherwise had taken place. The 

 immediate freezing of the material would have obviated all 

 this. 



WTien thoroughly frozen the tubes are removed, washed under 

 a tap to remove salt from the surface, gently warmed until 

 thawed, carefully wiped outside and around the mouth to remove 

 any remaining traces of salt and the contents folded in a small 

 square of extra heavy muslin cloth (which has been boiled through 

 three changes of distilled water and dried away from dust) for 

 thorough squeezing in a heavily tinned ''beef juice" press. The 

 sap is then either filtered through Schleicher and Schlill "Barium" 

 filter (No. 589) or better still centrifuged at high speed. The 

 removal of suspended solids by means of a centrifuge makes for 

 more rapid work and prevents the loss of the juice necessary to 

 wet the filter paper. 



The classical method of determining osmotic pressure by freez- 

 ing point lowering is that of Beckmann. Recently, other methods 

 have been suggested by Dixon- and followed by Ohlweiler.'' 



The slowness of the exceedingly simple Beckmann method 

 seems to us to have been over emphasized. The rapidity of 

 Beckmann determinations like that of any other piece of labora- 

 tory routine depends largely upon the skill and practice of the 

 operator and the care with which every detail of the work is 

 systematized. 



Mr. Lawrence, who did the bulk of our freezings, after a little 

 practice easily averaged over twenty in a half day. This was 

 possible by using two freezing tubes^ which were filled and 



2 Dixon, H. H., Sci. Proc. Roy. Dublin Soc. n. s. 12: 275-311 1910, 13: 49-62. 

 1911. 



3 Ohlweiler, W. W., Ann. Report Mo. Bot. Garden 23: 101-131. 1912. 



* The standard Beckmann freezing tube is supplied with a small side tube for 

 the introduction of additional quantities of solvent or of weighed amounts of the 

 solute under investigation, so that more than one determination of molecular 

 weight may be carried out without removing the thermometer. This extra open- 



