178 CHEMICAL DISCOVERY AND INVENTION 



SUGAR IN WATER 



Osmotic pressure 



Percentage P in mm. of 



of sugar, C. mercury. C 



1 .. 535 .. 535 



2 . . 1016 . . 508 

 2-74 .. 1513 .. 554 

 4 .. 2082 .. 521 

 6 .. 3075 .. 513 



It is evident in this case that, allowing for reasonable experi- 

 mental errors, the osmotic pressure is in the case of sugar directly 

 proportional to the amount of dissolved substance per unit 

 volume. 



POTASSIUM NITRATE IN WATER 



Percentage of Osmotic 



nitrate, C. pressure, P. C 



0-8 .. 1304 .. 1630 

 143 .. 2185 .. 1530 

 3-33 .. 4368 .. 1330 



p 



In this case the ratio ^ declines somewhat as the concentration 

 O 



of the liquid is increased, but this is evidently due to the passage 

 of a small quantity of the salt through the membrane. 



When a substance of any kind is dissolved in, say, water the 

 properties of the liquid are modified. Thus it is a matter of 

 common knowledge that sea water does not freeze so easily as 

 fresh water, and the practice is familiar of strewing salt on a 

 frozen surface to induce thaw, that is to form a liquid which 

 remains liquid, while water at the same temperature is ice. It 

 may not be so commonly known that the boiling-point of water 

 is raised many degrees by the addition of common salt, but this 

 is familiar to the practical chemist, who makes use of the fact 

 when he requires a bath for experimental purposes somewhat 

 hotter than boiling water. 



About 1883 the first tolerably accurate estimations of the 

 effect of dissolved salts in lowering the freezing-point of water 

 were published by the late Professor Raoult of Grenoble. It was 

 previously known that a determinate quantity of the same 

 substance dissolved in the same quantity of water always reduced 



