172 CHEMICAL DISCOVERY AND INVENTION 



Why the molecules of calcium carbonate are not separable 

 from one another in a similar way by the action of water is 

 usually explained by saying that the cohesion between them is 

 greater than the cohesion between molecules of sugar. That, 

 however, is merely a word and not an explanation, and serves 

 as an admission of ignorance as to the nature of the difference 

 between the two substances. 



Sugar dissolved in water is then assumed to be in a condition 

 comparable with that of a gas, only the spaces between the 

 molecules are occupied by moving molecules of another kind. 

 The molecules of gases exert pressure which is regulated by 

 temperature, and the number of molecules in a given volume 

 conforms to the law of Avogadro. In the solution of sugar an 

 analogous condition prevails. The boundary of a mass of liquid 

 is determined by what is called " surface tension," which acts in 

 such a way that the liquid, at any temperature much below its 

 boiling-point, behaves as though it were confined within an 

 elastic skin which always tends to squeeze it into the smallest 

 possible space. This is shown by the spheroidal form of detached 

 drops. Within this bounding surface the sugar and the water 

 exert a pressure which is called the " osmotic pressure." The 

 earliest observations of this pressure and measurements of it 

 were made by Pfeffer, professor of botany at Bale some thirty 

 years ago. But the interpretation of his results led the Dutch 

 professor Van 't HofI, in 1887, to formulate the theory of solution 

 which has just been briefly explained. Van 't Hon found from 

 Pfeffer's measurements the existence of a complete parallelism 

 between the osmotic pressure of a dissolved substance and the 

 laws which govern gas pressures. 



In the first place the osmotic pressure, at any rate in dilute 

 solutions, is in direct proportion to the strength of the solution, 

 that is, to the amount of dissolved substance in unit volume of 

 the liquid, and this is equivalent to saying that the osmotic 

 pressure is inversely proportional to the volume, which is one 

 form of Boyle's Law. 



PfefEer also found that osmotic pressure increases with tem- 

 perature and that the increase is in harmony with Gay Lussac's 

 law for gases, which states that the volume of a gas is directly 

 proportional to the absolute temperature. 



Further, when solutions containing different substances of the 

 same chemical character are compared together and the quanti- 



