EXPERIMENT STATION BULLETINS. 519 



The cleterniinatioii of the osmotic pressure is possible directly and in- 

 directly. The former method is tedious and too specific and not adapted 

 to our purposes. Of the indirect, the cryoscopic, or freezing point method 

 of Beckmann was adopted because of its simplicity and reliability. 

 As a complete description of this method can be found in most text- 

 books on physical chemistry, only a few points will be touched. In our 

 work the thermometer was standardized each day by the freezing 

 of distilled water before and after data were collected on the solution. 

 To prevent any loss of mercury by its rise in the column after removal 

 from the frozen solutions, the thermometer was kept in a tube sur- 

 rounded by the freezing mixture. It is a well-known fact that solutions 

 freeze at a lower temperature than the solvent itself. The amount of 

 this depression depends upon two factors, — concentration of the sub- 

 stance dissolved and the kind of substance in solution. Two solutions, 

 each containing the same quantity of different substances, will have the 

 same concentration, but their depression of the freezing point will vary 

 on account of the different dissociative properties of the substances in 

 the solution. However, the depression of the freezing point may not 

 give the kind of substance dissolved, but will give an idea of the solu- 

 tion pressure exerted, — Avhether there is a single substance or a mix- 

 ture of substances present. Upon this principle of lowering the freez- 

 ing point is based the determination of the molecular weight and os- 

 motic pressure since both depend upon the concentration of the solu- 

 tion. The figures in the column for osmotic pressure are in degrees, the 

 amount of depression of the freezing point. These can be easily changed 

 to atmospheres by referring to the table in the appendix. 



Specific conductivit}', the reciprocal of the resistance, is important in 

 that it gives the concentration of the solution, especially the ion con- 

 centration, although it does not give any indication as to the presence 

 of colloids or non-electrolytes. The colloids may carry a charge of 

 electricity either as a colloid itself or due to the electrolytes adsorbed 

 by it. The Kohlrausch roller type bridge with a telephone as a detector 

 of the minimum sound given by the induction coil is used for obtaining 

 the resistance of the solution. The current is furnished by a 2-volt stor- 

 age battery. As the resistance varies according to the area of the elect- 

 rodes and distance between them, the relation between the area and dis- 

 tance is determined with a known solution of N/50 KCl. This ratio is 

 used for placing the result obtained for the unknown solution upon a 

 comparable basis. 



The reaction of the soil is a very important factor both in case of the 

 microorganisms and of the plants. Many times the solution is so col- 

 ored that it is impossible to use an indicator. The solutions are titrated 

 by the conductivity method. On account of the difficulty of determining 

 the exact neutral point of the ammonium citrate solution by the regular 

 method, Hall and BelP, and Patten and Robinson- independently applied 

 the conductivity method as a more accurate means of its determination, 

 Avhile Kiister and Griiter'' used this method for finding the end point of 

 a mixture of weak acids and bases and also in colored liquids where it 

 was impossible to use indicators. 



^IlaU, R. A. and Bell, .T. M. Physical Properties of Aqueous Solutions Containing Am- 

 monia and Citric Acid. .Tour. Am. CJiem. Soc. S'.^, pp. 711-S. 1911. 



-Patten, A. J. and Robins )n. C. S. Neutral Ammonium Citrate Solution. Tech. Bui. No. 

 12, Mich. Agr. Expt. Sta., 1912. 



■■'Kiister. P. W. and Griiter, Max. Uober die restleguns dcs Neutralisation punklis durch 

 Leitfiihiglseitamessung. 2. Anorg. Chem. 35, p. 454, 1903. 



