146 ELEMENTS OF SCIENCE 



There is also a peculiar action of liquids which may 

 here be mentioned. If two liquids of different densities 

 are placed within a vessel so that they are separated by 

 a median porous partition, then a portion of each liquid 

 will pass through the partition, but more of the less 

 dense liquid will pass through it than of the other. 

 The consequence is that if the level of the two liquids 

 be at first the same on each side of the partition, then 

 the level of the denser liquid will rise, while that of the 

 less dense liquid will sink. This process of fluid trans- 

 ference is called " osmosis," and it is facilitated if the 

 partition be itself a colloidal substance. 



We must now return from this digression (which has 

 arisen from what it was necessary to say concerning 

 crystals) and consider a little further what are the solid, 

 liquid, and aeriform bodies which compose this earth. 



Limestone, marble and chalk, all consist of carbonate 

 of lime* and have been produced by the chemical union 

 of lime and carbonic acid. If sulphuric -acid be poured on 

 any of the three substances, or if small pieces of them 

 be placed in a solution of sulphuric acid, bubbles will be 

 given off there will be effervescence. This is due to 

 the acids changing places. The sulphuric acid unites 

 with the lime, or " base,"f and the carbonic acid, which 

 is normally a gas, is set free hence the effervescence. 

 Other acids e.g., strong vinegar will produce a similar 

 effect. Soil, stones, and rocks which can be thus acted 

 on are called calcareous. Soil, stones, and rocks, where 

 flint or silica plays the part which Jime plays in the 

 calcareous rocks, are termed silicious. Such are ala- 

 baster, slate, sand and sandstone rocks. Acids have no 

 effect upon them. Silicious crystals are extremely hard, 



* See ante, p. 140. t See ante, p. 134. 



