24 SCIENCE PROGRESS. 



temperature range over which they extend is but some 

 30°, the pressure alteration being only 6*2 cm., the 

 results do not furnish very decided support to the law. 



By the use of blood corpuscles, Donders and Ham- 

 burger found that solutions — for the most part electrolytic 

 — which were isotonic at o° were still isotonic at 34 . 

 This result indicates that the effect of temperature on 

 osmotic pressure is the same for solutions of different 

 substances ; it gives no clue, however, as to the law of 

 the temperature change. It is evident that the experi- 

 mental support to the application of Gay Lussac's law 

 to dilute solutions cannot be regarded as very strong. 

 The range over which the observations extend is too 

 short to admit of the accurate determination of the law 

 expressing the variation of osmotic pressure with the 

 temperature. 



Avogadrd s hypothesis as applied to dilute solutions — 

 According to this hypothesis equal volumes of solutions 

 which at the same temperature have the same osmotic pres- 

 sures should contain the same number of molecules, and 

 consequently equimolecular solutions should be isotonic. 

 De Vries was the first to show that this was the case. By 

 his methods he found the isotonic molecular concentrations 

 of two substances A and B, that is, the concentrations for 

 equal osmotic pressures. Assuming the proportionality of 

 osmotic pressure and concentration, the reciprocal of the 

 ratio of the above concentrations is the ratio of the 

 osmotic pressures of A and B for equimolecular concen- 

 trations. De Vries took potassium nitrate as the standard 

 substance B, and put its osmotic pressure equal to 3, 

 and the osmotic pressure of a substance referred to that 

 of nitre as 3 he termed its isotonic coefficient. The 

 isotonic coefficients of indifferent solutions should, ac- 

 cording to the hypothesis, be the same. Very few 

 indifferent substances have been investigated. The three 

 methods (I., II., III.) used by De Vries gave the values 

 for the isotonic coefficients represented in the table. In 

 column IV. is a result obtained by the use of blood 

 corpuscles. 



