428 Bemarlcs and Observations suggested 



which were added to the old soil were ready to pull a year 

 sooner, in half the time of the rest of the ground ; this could 

 not arise from fallen leaves, as the belt was mostly all of Scotch 

 firs ; and so perceptible was the difference between the old and 

 new ground, that we could tell the site of the plantation for 

 several years by its effects on all the nursery plants that were 

 grown on it. Most potted plants will grow in very different 

 composts, but require different treatment : in light dry soils they 

 must be more frequently watered, and kept more cool ; when 

 more loam is in the compost, they need less attention, and are 

 more hardy, but grow more slowly. For the other great benefits 

 derived from bottom heat, the necessity of suiting the temper- 

 ature of the soil to that of the atmosphere, the disadvantages of 

 neglecting this in the growing of grapes, the great advantages of 

 keeping the roots warm in forcing, &c. Sic, recourse must be had 

 to the work itself. 



On the analogous sections, of Temperature, and Protection 

 from Frost, the great effort should be to retain, if possible, the 

 heat which was accumulated near the plants through the day. 

 If water be near, it has a tendency to assume the state of vapour, 

 and rob the air of its heat ; the sap of the plant may be more 

 abundant also, from this cause, and increase the expansion of 

 the fluids by frost, which may end in the bursting and laceration 

 of the vessels, and be the cause of death. When a clear cold 

 night succeeds to a wet day, if the night is long, and the at- 

 mosphere does not get cloudy, the heat radiates upwards from 

 the earth and plants into the cold air ; while the evening at first 

 is comparatively warm. The cold is also greatly accelerated by 

 the evaporation of moisture: it is calculated that it takes above 

 800*^ of heat to convert water into steam; and though va-| 

 pour does not require so much, part of the vapour being 

 chemically attracted by the atmosphere, still the consumption is 

 gi'eat. From these causes the earth and plants by degrees get 

 so cold, from having parted with their heat, that their temperature 

 descends below the freezing point. In spring and autumn the air 

 is comparatively warm, and the nights not so long ; and hence 

 spring and autumn frosts seldom take place till near sunrise: 

 and if a cloud happens to settle above any portion of the earth 

 about that time, before the earth has been cooled down to the 

 freezing point, it prevents the further radiation of the heat up- 

 wards; and hence we often find places lying contiguous and; 

 below the cloud to be saved from frost at one time, while at] 

 another they will be much hurt. Where plants partially cover 

 one another, they help to prevent radiation ; and when one 

 plant is more covered with moisture than another, or growing! 

 more vigorously, more full of watery sap, and the bark morej 

 tender, from these and other causes one plant is often, to all 



