637 



the air and, as Stockbridge^ proves, can arise as vapor during the summer 

 months from the soil which in the night is warmer than the air. If there 

 is a strong dew covering, it can be considered rather as a means of protec- 

 tion against the freezing of the plants. If this dew freezes, a crystalline 

 coating is produced which is identical with the ice covering. Hoar frost, 

 on the other hand, is produced when the point of condensation of the air 

 lies below zero degrees. This degree of temperature is reached through 

 radiation and evaporation from the plant; therefore, the mist molecules 

 attach themselves to one another in a firm crystalline form (soil or summer 

 hoar frost). The covering of frozen mist, or winter hoar frost, is pro- 

 duced by the flowing of the equatorial current into the slowly displaced 

 polar current; the change is dangerous because, with longer duration, so 

 thick a covering of frozen mist can be produced that the strongest trees 

 break under its load. 



In nurseries, the prompt and careful beating of the branches with 

 sticks will prevent such an injurious accumulation of ice. This naturally 

 cannot be carried out in forests. 



In summer frosts, the cultural conditions are often of decisive signifi- 

 cance. It should be taken into consideration, in tilled soil, that the plant 

 body cools down more rapidly than does the soil which, in the night, acts as 

 an equalizing source of heat and prevents, more or less, the formation of 

 hoar frost. This effect will be the greater, the larger the water content of 

 the soil which thus retards the cooling down. On damp fields the dew, 

 which moderates the cooling of the leaves, is formed earlier and more 

 abundantly than on dry soils. On the other hand, cultural regulations 

 which prevent the rising of heat from the drier soil layers, such as the 

 loosening of the soil, or a strawy manure, favor frost". 



1 Journal of science, Vol. I, p. 471; cit. Naturforscher 1879, No. 32. 



2 Petit, M., Einfluss einiger Kulturverfahren auf die Bildung- von Reif. Anna!, 

 agron. 1902, No. 7, cit. Centralbl. f. Agrikulturchemie 1903, p. 557. 



