t 
ELEMENTS OF SOILS. 225 
indicates that the atmosphere does not extend beyond forty-five miles from the surface 
of the earth, although some other phenomena would lead 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 absorbed 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 earth, 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 the 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 rays, 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 sufficient 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 
