38 AGRICULTURE. 



limestone bridge where the rains beat upon it, we notice that 

 where the water trickles down, some of the limestone has been 

 washed out, and, in some places, long stone " icicles " have 

 formed. The limestone has been dissolved out by the car- 

 bonic acid in the water. Water in the soil contains some 

 carbonic acid, and the air contains carbonic acid gas ; so that 

 we have in this an explanation of the hollowing out of caves in 

 limestone rocks, the breaking down of limestone cliffs, and 

 the rapid changes that take place in limy soils. 



EFFECTS OF DRAINING. We take up a handful of vegetable 

 soil swamp muck, for instance, or wood mold it is easily 

 ground up between the fingers ; there does not appear to be 

 much rocky or sandy material in it. If we shake it up in a 

 bottle of water, we find that the water becomes more or less 

 brown in color ; some of the substance has dissolved, but only 

 a little. In order to get this material into a soluble form, the 

 air must be allowed to work upon it. But the air cannot get 

 into it unless it is drained. 



Take two tin cans or tight boxes ; fill one with wet muck from an un- 

 drained, swampy field, and fill the other with dry leaf mold. Plant a few 

 seeds of the same kind in each, and observe how much better the dry, 

 well -aired leaf mold is for the growing of valuable farm plants than the wet 

 swamp muck. 



Wet, swampy soil needs first to be drained and then to be 

 well worked over, so that the air can get in through it to 

 weather it There is another reason for letting the air into the 

 wet, swampy soil, and that is, it will sweeten it. Vegetable 

 soils that are watec-logged are sour, or acid ; and seeds will not 

 sprout nor plants grow well in sour soils. The air contains 

 some ammonia, and this, when it gets into the soil, changes it 

 from a sour to what we may call a sweet soil it takes the 

 sourness out of it If a little lime be scattered over the 

 drained soil, this sweetening will be hastened. 



Then, again, wet, swampy soils are usually cold, because of 

 the water that they contain. When we wish to cool a room on 



