ACTION AND PROPERTIES OF SNOW. 37 



exposes them ;* and when a thaw arrives, by slowly melling, it allows 

 the tender herbage gradually to accustom itself to the milder atmosphere. 



In this manner there is no doubt that a fall of snow may often be of 

 great service to the practical farmer. But some believe that winter 

 wheat actually thrives under snow. On this point I cannot speak from 

 personal knowledge, but 1 will here mention two facts concerning snow, 

 which may possibly be connected with its supposed nourishing quahty. 



In the first place, snow generally contains a certain quantity of ammo- 

 nia, or of animal matter which gives off ammonia during its decay. 

 This quantity is variable, and is occasionally so small as to be very dif- 

 ficult of detection. Liebig found it in the snow of the neighbourhood of 

 G lessen, and I liave this winter detected traces of it in the snow which 

 fell in Durhamf during tv.'o separate storms. This ammonia is present 

 in greater quantity in the first portions that fall and lie nearest the plant. 

 Hence if the plant can grow beneath the snow, this ammonia may affect 

 its growth ; or when the first thaw comes it may descend to the root, and 

 may there be imbibed. Rain water also contains ammonia, but when 

 rain falls in large quantity it runs off the land, and may do less good than 

 the snow, which lies and melts gradually. [For the properties of am- 

 monia, see Lecture III:] 



Another singular property of snow is the power it possesses of ab- 

 sorbing oxygen and nitrogen from' the atmosphere, in proportions very 

 different from those in which they exist in the air. The atmosphere, as 

 already stated, contains 21 percent, of oxygen by volume (or bulk), but 

 the air which is present in the pores of snow has been found by various 

 observers to contain a much smaller quantity. Boussingault [Annalen 

 derPhysick (Poggendorf), xxxiv., p. 211,] obtained from air disengaged 

 by melting snow 17 per cent, of oxygen only, and De Saussure found 

 still less. The difficulty of respiration experienced on very high moun- 

 tains has been attributed to the nature of the air liberated from snow 

 when melted by the sun's rays. Whether the air retained among the 

 pores of the snow, which in severe winters covers our corn-fields, be 

 equally deficient in oxygen with that examined by Boussingault, and 

 whether, if it be, the abundance of nitrogen can at all affect vegetation, 

 are matters that still remain undetermined. 



II. In the fluid state, that of water, the agency of tliis compound in 

 reference to vegetable life, though occasionally obscure, is yet every- 

 where discernible. 



Pure water is a colourless transparent fluid, destitute of either taste or 



* The effects of such alternations are seen on the occurrence of a night's frost in spring. 

 If the sun's rays fall in the early morning, on a frozen shoot, it droops, withers, and black- 

 ens—it is destroyed by the frost. If the plant be in a shaded spot, where the sun does not 

 reach it till after the whole atmosphere has been gradually heated, and tlie frozen tissue 

 slowly thawed, its leaves sustain little injury, and the warmth of the sun's rays, instead of 

 injuring, cherish and invigorate it. This effect of sudden alternations of temperature on or- 

 ganic matter explains many phenomena, to which it would here be out of place to advert. 



A thick light covering of porous earth not beaten down preserves the potatoe pit from the 

 effects of the frost better than a solid compact coating of clay, in the same way as snow 

 protects the herbage better than a sheet of ice; and it is because of the porosity of the 

 covering, that ice may be preserved more effectually, and for a longer period, in a similar 

 pit, than in many well-constructed ice-houses. 



t By adding two drops of sulphuric acid to four pints of snow water, evaporating to dry- 

 ness, and mixing the dry mass with quicklime or caustic potash. The residual mass con- 

 tained a brown organic matter, mixed with the sulphate of ammonia. 



