SIZE OF PARTICLES 43 



chemical investigations and determinations on other chemical 

 compounds of benzoic acid. 



From similar considerations the minimum value for the molecular 

 weight of certain albumins are obtained. If a protein contains one 

 per cent of sulphur, then its molecular weight must be 3200 times 

 heavier than that of hydrogen. (The atomic weight of S = 32.) 

 But there is every reason to believe that there are at least two 

 atoms of sulphur in egg albumen, because one-half of the sulphur is 

 easily split off, whereas the other splits off with difficulty. Thus egg 

 albumen, with one and three-tenths per cent sulphur shows a molec- 

 ular weight of 4900, and oxyhemoglobin a molecular weight of 

 14,800. Oxyhemoglobin contains 0.4 to 0.5 per cent of iron; pro- 

 vided it contains but one atom of iron, its molecular weight must be 

 11,200 to 14,000. The figures obtained approach one another very 

 closely. 



Another method of obtaining the molecular weight is based on 

 Avogadro's law. This law says: "At the same temperature and 

 equal pressure, different gases contain the same number of molecules 

 per liter." Thus from the weight of a gas or of a vaporized sub- 

 stance, the molecular weight can be determined, if we compare its 

 weight with that of an equal volume of hydrogen gas. Avogadro's 

 law was generalized by J. H. VAN'T HOFF and extended to solutions. 

 According to this generalization the "osmotic pressure" of a dis- 

 solved substance is proportionate to the number of the dissolved 

 molecules and is as large as if the substance were vaporized. If a 

 sugar solution is placed in a porous clay cell which is so dense that 

 water but not sugar may pass in and out, 1 the sugar tries to expand 

 like a gas and, as a result, water enters the cell and the solution rises 

 in it. If a vertical tube has been attached to the cell, the osmotic 

 pressure of the solution may be measured directly from the height to 

 which the fluid rises. There are indirect methods also, the under- 

 lying principles of which we cannot discuss here. They depend on 

 the fact that in proportion to the osmotic pressure the boiling point 

 is raised and the freezing point lowered. In ideal cases these changes 

 are strictly proportionate to the concentration of the dissolved sub- 

 stance in just the same way that the original volume of an ideal 

 gas is reduced to one-half by double the pressure and to one-third 

 by three times the pressure. Consequently by determining the 

 freezing or boiling point of a solution, molecular weight may be de- 

 termined. In the case of crystalloids this method is preferred to the 

 direct reading of the osmotic pressure for the following reasons: It 

 is exceedingly difficult to prepare a cell that really holds back crystal- 

 1 Such a chamber is said to be semipermeable. 



