Dixon and Atkins— 0« Osmotic Pressure in Plants, tVc. 287 



it and the freezing-point. This relation has been determined experimentally 

 and also deduced theoretically. Nernst gives the equation ' : — 



A X 12'03 = P in atmospheres. 



In 'column M is fouUd the calculated mean molecular weight of tlie 

 dissolved substances causing tlie depression of freezing-point. To obtain this 

 it is necessary to ascertain the weight of the solvent and tlie dissolved 

 substances in the solution. These data are obtained by filtering and 

 weighing the sap after its freezing-point has been determined, by evaporating 

 to dryness in a steam-oven and weigliiug the residue. M is then obtained by 

 the formula^ : — 



In this s = weight of solute, and L weight of solvent, viz. tlie weight of 

 the sap mimes the weight of the dissolved substances. 



Great accuracy is not to be expected from this method of determining the 

 molecular weight ; but it is hoped that by its means an indication of some 

 value has been obtained of the composition of the sap. Where the mean 

 molecular weight is much above 200, it seems reasonable to suppose a 

 disaccharide such as saccharose (mol. wt., 342) or maltose is a large con- 

 stituent of the dissolved substances, while, when the molecular weight is 

 found to be about 150, these bodies cannot be present in any large proportion. 



1 W. Nernst, Theoretical Chemistry. English Translation of Fourth Edition. Macmillan, 

 London, 1904, ]). 144. 



• James Walker, Introduction to Ph3sical Chemistry. Macmillan, Loudon, 1899, p. 186. 



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