C 186 ] 



in northern winters is usually very much below the 

 freezing point ; and this water becomes the first nour- 

 ishment of the plant in early spring. The expansion 

 of water during its congelation, at which time its 

 volume increases tV, and its contraction of bulk dur- 

 ing a thaw, tend to pulverise the soil ; to separate its 

 parts from each other, and to make it more permeable 

 to the influence of the air. 



If a solution of lime in water be exposed to the 

 air, a pellicle will speedily form upon it, and a solid 

 matter will gradually fall to the bottom of the water, 

 and in a certain time the water will become tasteless ; 

 this is owing to the conbination of the lime, which 

 \vas dissolved in the water, with carbonic acid gas 

 which existed in the atmosphere, as may be proved 

 by collecting the film and the solid matter, and ignit- 

 ing them strongly in a little tube of platina or iron ; 

 they will give off carbonic acid gas, and will become 

 quicklime, which added to the same water, will again 

 bring it to the state of lime water. 



The quantity of carbonic acid gas in the atmos- 

 phere is very small. It is not easy to determine it 

 with precision, and it must differ in different situa- 

 tions ; but where there is a free circulation of air, it 

 is probably never more than - 5 ^, nor less than -5*0 of 

 the volume of air. Carbonic acid gas is nearly j hea- 

 vier than the other elastic parts of the atmosphere in 

 their mixed state : hence at first view it might be sup- 

 posed that it would be most abundant in the lower 

 regions of the atmosphere j but unless it has been 

 immediately produced at the surface of the earth in 



