ELEMENTS OF SOILS. 225 



indicates that the atmosphere does not extend beyond forty-five miles from the surface 

 of the earth, althoiigli some other phenomena would h.'ad us to infer that it extends much 

 farther. 



The sun's rays, in passing through the atmosphere, do not impart to it a sensible amount 

 of heat. They pass on to the earth and are absorlied by its surface, whence the heat again 

 issues by radiation, and warms the lowest stratum of the atmosphere, which ascends and 

 communicates its heat to the other layers in succession. By contact with the eartli, then, the 

 air is heated ; and the farther it is removed from the surface, the less caloric it receives, 

 till at a certain height the uniform temperature is reduced to 32°. The height at which 

 this effect occurs, depends upon the quantity of heat which the earth receives from the 

 sun. This is greatest at the equator, and hence the point of perpetual congelation is the 

 highest there. Thus, at the equator, this point is 15,000 feet above tlic level of the sea ; 

 and from the equator it constantly approaches the earth, until at the poles it sinks below 

 the surface. 



The relations of the atmosphere to heat, form one of its most important properties. Air is 

 ranked among the non-conductors. When confined in a space, it prevents the escape of 

 heat. If it was capable of being heated by the transmission of the sun's ra3s, it would 

 render the earth uninhabitable. 



Ammonia. This compound of nitrogen and hydrogen is exceedingly important in ve- 

 getation. Some of our most important grains require its presence. It exists in the 

 atmosphere ; and it is developed in the decay of animal and vegetable substances, from 

 which it escapes into the atmosphere, ready to enter into new combinations. One single 

 property of this substance fits it to play its important part in the vegetable economy, namely, 

 its ready absorption by porous bodies. This property is manifested and proved in innume- 

 rable instances, some of which fall under observation in our ordinary manual operations ; 

 for example, plaster, when placed in a stable, or in any place where organic matters are 

 undergoing decomposition, takes up the ammonia as it escapes : lime also performs a 

 similar office. A direct experiment, which proves this statement, is often performed in the 

 laboratory ; thus, we have only to pass a little plaster, lime, charcoal, earth, etc., into a 

 receiver containing ammonia over mercury, when the whole of the ammonia disappears : 

 it is absorbed and condensed in the pores of the body employed. Any moist substance 

 whatever produces this effect instantaneously, so powerful is the affinity of ammonia for 

 water. The same process goes on in nature : the ammonia floating in the atmosphere is 

 continually absorbed by soils, by humus, and especially by clay ; and all these substances 

 give out their ammonia on the application of sufficieni heat to dissipate their water. Ex- 

 posing fresh surfaces of soil to the air, is one means of procuring a fresh supply of this 

 matter. Clay, and the oxide of iron contained in the soils, perform the important function 

 of absorption. This property of clay is the one which renders clay soils so much better for 

 wheat, than sandy soils : it furnishes a supply of ammonia, from which the wheat forms its 

 nitrogenous matters. 



[Agricultural Report.] ' 29 



