118 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



the ratio of osmotic to gas pressure. In all other concentrations the 

 ratio approaches unity. 



The striking agreement between the observed osmotic and theoretical 

 gas pressure, which is seen in Tables 10 and 13, gave the author, for 

 a time, much more confidence in the trustworthiness of these first 

 results than they were afterwards found to deserve. The evidence 

 furnished by them appeared to confirm the conclusions of van't Hoff 

 regarding the measurements of Pfeffer. The solutions employed were, 

 in general, much more concentrated than those of Pfeffer, and more 

 concentrated also than those to which van't Hoff restricted his de- 

 ductions regarding osmotic pressure; but it was believed that, by the 

 adoption of the "weight-normal" system, the term b in the van der 

 Waals equation had been practically eliminated for osmotic pressure. 

 There was, therefore, no apparent inconsistency in the seeming con- 

 formity of concentrated as well as dilute solutions to the formula of 

 van't Hoff. The correctness of this view of the function of the weight- 

 normal system will be maintained later by means of data whose validity 

 can not be questioned. The real ground for suspecting the trust- 

 worthiness of the results of Series I was revealed by the polariscope 

 in connection with the work of Series II. 



Series II.* 



The measurements of Series II were made under more favorable 

 conditions than those of Series I. Some of the improvements which 

 were introduced will be enumerated: 



1. In the earlier work there had been much uncertainty as to the 

 exact capacity of the upper end of the manometer, where the form 

 of the tube had been altered in closing the instrument in the flame. 

 The original calibration could not hold for this part of the manometer, 

 and there was no obvious method of ascertaining its capacity directly. 

 It had been customary, therefore, to measure the height of the affected 

 part and to assign to it a spherical, or a conical, or a conico-spherical 

 form, according to its appearance. The diameter of the bore at the 

 base was known from the calibration. This method would have 

 sufficed if the form had been strictly spherical or purely conical; but, 

 as a rule, it was neither the one nor the other, but a mixture of the 

 two, and it was necessary, in estimating capacity, to guess in what 

 proportion each form was represented. It could be easily proved, by 

 examples of the effects of minute errors in manometric work, that the 

 problem was one of great importance. It was solved satisfactorily 

 by filling the upper end of the manometer with mercury in the manner 

 described in a previous chapter. 



♦Measurements by H. N. Morse, J. C. W. Frazer, E. J. Hoffman, and W. L. Kennon. Am. 

 Chem. Jour., xxxvi, 39. 



