CULTIVATION. 195 



Tlie presence of too miicli water, by excluding the 

 oxygen of the air, prevents the comminution of min- 

 eral m^atters necessary to fertility. 



IG. The evaporation of water ^ and the consequent 

 cooling of the soil, is in a great m^easure prevented 

 by draining the water out at the hottom of the soil, 

 instead of leaving it to be dried oil' from the sur- 

 face. 



When water assumes the gaseous (or vapory) form, 

 it occupies nearly 2000 times the space it occupied 

 as a liquid, and as the vapor is of the same tempera- 

 ture as the liquid, it follows that it contains vastly 

 more heat. A large part of this heat is derived 

 from surrounding substances. When water is sprink- 

 led on the floor, it cools the room ; because, as it 

 becomes a vapor, it takes heat from the room. The 

 reason why vapor does not feel hotter than liquid 

 water is, that, its heat is diffused through the larger 

 mass, so that a cubic inch of vapor, into which M-e place 

 the bulb of a thermometer, contains no more heat than 

 a cubic inch of water. The principle is the same in 

 some other cases. A sponge containing a table- 

 spoonful of water is just as wet as one twice as large 

 containing two spoonfuls. 



If a wet cloth be placed on the head, and the evap- 

 oration of its water assisted by fanning, the head 

 becomes cooler — a portion of its heat being taken to 

 sustain the vapory condition of the water. 



The same principle holds true with the soil. 

 When the evaporation of water is rapidly going on, 

 l)y the assistance of the eun, wind, etc., a large 



