PLANTS, WITH GLASS ROOFS. 181 



the modes of heating employed. Now, however, that it begins to be 

 well known that plants derive a great proportion of their carbon from 

 the air, another and the most important use of ventilation has been 

 discovered ; and gardeners are become aware that a constant supply of 

 fresh air is almost as necessary to plants as water, and, consequently, 

 that without fresh air no plants can be kept in a perfectly healthy and 

 vigorous state. The admission of air for the purpose of nourishing 

 plants has been very properly distinguished by Dr. Lindley under the 

 name of Aeration, from ordinary ventilation, and it requires to be 

 regulated in quite a different manner. It has been already observed 

 that if the sashes of a hothouse are opened in front and in the upper 

 part of the roof at the same time, so as to create a thorough draught, 

 Avlien the atmosphere is colder than the temperature of the house, a 

 great injury is done to vegetation, not only by the sudden chill, which 

 the admission of a current of cool air produces, but by the quantity 

 of moisture which it carries off. Hence, aeration should be effected 

 by the circulation of a constant supply of warm moist air ; and hence 

 it is that plants grown in houses heated by the Polmaise system are 

 generally in a state of vigorous health. Ventilation is, however, 

 frequently necessary as well as aeration. In greenhouses, pits, and 

 frames, where there is a large proportion of earthy and moist surface 

 to a small volume of air, the latter may become too moist, and fresh 

 air may be required to dry it ; and in every description of plant- 

 structure it may be required to lower the temperature. Hence, for 

 houses heated by smoke-flues, and for pits and frames heated by fer- 

 menting dung, a greater power of ventilation becomes requisite than 

 for houses heated by hot water, in which noxious vapours can rarely 

 be produced, or the temperature raised much above 80 or 90. For 

 lowering the temperature of a hothouse, air is best admitted by 

 opening sashes or ventilators in the upper part of the roof. In roofs 

 with sliding sashes, the upper sashes along the whole line of roof may 

 be let down uniformly, if the house be at an equal temperature 

 throughout, and rather more at the hottest part, if it is of unequal 

 temperature. The width opened need seldom exceed half an inch or 

 an inch in the winter time ; but in summer it may be much greater, 

 according to the temperature to be kept up in the house, and other 

 circumstances. If the roof should be a fixed one, then a narrow 

 opening might be made in the upper angle of the roof along the whole 

 length of the house, and the cover to this opening might be raised 

 simultaneously and uniformly by simple mechanical means. A 

 portion of the heated air of the house will escape by this opening, 

 while a portion of the outer air will enter to take its place, mixed, as 

 it descends, with the heated air, and becoming by this means heated to a 

 oTtain extent before it reaches the plants. The great object in venti- 

 lating houses which are kept at a high temperature is to avoid thorough 

 draughts, which are always produced when ventilators in the front 

 and back are opened at the same time. Even in houses kept at a low 

 temperature, such as greenhouses and conservatories, it is desirable in 

 the winter season to admit the air from the roof only, and not from 



