CAUSED BY EXTERNAL COLD. 209 



must have leaked ; and gas must have escaped into the house ; yet the 

 house is warmed with water in pipes. In that case, it must have been 

 something from the pipes ; or a hole must exist between the inside of the 

 Vinery and the furnace, or the stoke-hole. Finally it is suspected that 

 the Vines must be poisoned by something in the soil, or in the water. 

 But in all such cases the injury proceeds from dryness and nothing else. 

 If a piece of cold glass is introduced into a warm damp room, moisture, 

 in the form of dew, is immediately formed upon the glass. This mois- 

 ture previously existed in the form of elastic vapour ; ■ and the dew that 

 forms on the glass is at the expense of the vapour surroimding it ; so 

 that if a cubic body of such vapour be represented by 1000, and 250 

 parts of it be abstracted by precipitation, or condensation, upon the cold 

 glass, it is clear that there wiU be only 750 parts left ; in other words, 

 the air wiU. be one-quarter drier after the introduction of the cold glass 

 than it was before. Now, if we suppose that this abstraction of vapour 

 were carried to its utmost limits, it is evident that the end would be the 

 total drying of the air, in consequence of the condensation of all its 

 vapour on the surface of cold glass. The Vine leaf, when young, wiU 

 not bear an atmosphere the degree of saturation of which is much below 

 800. If the saturation falls to 500, it will be dried up and perish. 

 This being so, the force of Daniell's remarks wiU be obvious. And when 

 it is considered that a temperature at night of 20° is no very unfrequent 

 occurrence in this country when the saturation of the air may fall to 

 IWP, that is to say, instead of the atmosphere surrounding Vine leaves 

 amounting to 7 or 8 parts in 10, which is what they require, it may not 

 amount to more than \\ in 10, which is fatal to them, the amount of 

 the danger becomes still more striking : — Thus, in an atmosphere of 

 heat and moisture, the Vine leaves may actually die of drought"; and 

 that this occasionally happens, and is a frequent though unsuspected 

 cause of injury to tender foliage, near a glass roof, is perfectly certain. 



It is evident that the mode of preventing this drying of the 

 air by the cold surface of a glass roof will be, either by raising 

 the temperature of the glass, which can only be effected by 

 drawing a covering of some kind over our houses at night, so 

 as to intercept radiation, or by double glass sashes ; or else by 

 keeping the temperature of the air of the house as low as 

 possible, consistently with the safety of the plants, and so 

 diminishing the difference between the temperature of the 

 external and internal air. 



The objections to external coverings are, 1st, their expense, and, 2nd, 

 the trouble attending them. If, however, the needless cost of fuel, and 

 the injury sustained by plants, be placed in one scale, and the expense 



