ABSORPTION. 



hydrostatic pressure which exists when equilibrium is established; 

 with 100 grams of water, containing 6 grams of sugar, the osmotic 

 pressure was 3075 millimeters of mercury. 



Boyle- Van't Hoff Law. At constant temperature the osmotic 

 pressure of dilute solutions is proportional to the concentration of 

 the dissolved substance. Gay-Lussac- Van't Hoff law for dilute solu- 

 tions is as follows: At constant volume the osmotic pressure of 

 dilute solutions increases as the temperature; or, also, the osmotic 

 pressure of dilute solutions is proportional to the absolute temperature. 



Law of Avogadro-Van't Hoff. At the amc osmotic pressure 

 and the same temperature equal volumes of dilute solutions con- 

 tain the same number of molecules. The gases have been shown 

 long ago to have the same laws. Although osmotic pressure can be 

 obtained by the Pasteur-Chamberland cell with a deposit of copper 

 ferrocyanide in its pores, yet this determination is inaccurate; hence 

 we have recourse to the determination of the freezing point. 



According to Arrhenius, the dissociated ions of an electrolyte 

 in solution are capable of exciting pressure as well as the undis- 

 sociated molecules. 



It has long been known that the freezing-point of water is low- 

 ered by the addition of soluble substances. The lowering is, within 

 certain limits, proportional to the concentration of the solution. 



For the biologist the great importance of the freezing-point 

 determination lies in the fact that it enables him to ascertain the 

 number of molecules dissolved in a given volume of any body fluid. 

 A depression of the freezing-point of % oo degree corresponds to 

 an osmotic pressure equal to 0.012 atmospheres. While chemical 

 analysis can tell us much concerning the composition of physiolog- 

 ical fluids, it cannot yield us anything definite concerning the 

 osmotic behavoir of such solutions. This becomes intelligible when 

 we remember that the osmotic pressure of a solution is dependent: 

 upon the number of molecules (+ ions) it contains, and that this 

 cannot be determined by chemical analysis. By the determination 

 of the lowering of the freezing-point (cryoscopy) we have a direct 

 means of accomplishing our end. By finding out the freezing-point 

 of blood and of .urine it is possible to discover a lessened permeabil- 

 ity of the kidneys for dissolved molecules and disturbances in the 

 secretion of water. 



The freezing-point is determined by Beckman's differential 

 thermometer. This, the freezing-point of blood-serum of mam- 

 mals is 0.56 C. lower than water. It is usually expressed by the 



