35 



ters, such as sulphurous acid and ammonia, and the 

 consumption of carbonic acid, render ventilation essen- 

 tial to the health of vines, and of all plants in hot- 

 houses. They cannot inhale air overloaded with these 

 contaminations without being speedily injured, and 

 the proportions of those gases which rapidly cause 

 disease or even death, are much less than the gardener 

 usually suspects ; for if the sulphurous acid amounts 

 to no more than one cubic foot in ten thousand of 

 the air in a hothouse, it will destroy most of its in- 

 habitants in two days. To avoid such destruction, 

 for the comfort of visitors, and above all for the sake 

 of the plants' vigour, air should be admited as freely 

 as the temperature will permit. The foul warm air 

 can be easily allowed to escape through ventilators in 

 the most elevated parts of the roof, and fresh warm 

 air can be as readily supplied through pipes made to 

 enter near the flooring of the house after passing 

 through hot water, or other source of heat. 



We are quite aware that Mr. Knight has stated 

 that he paid little attention to ventilation, and that 

 plants will be vigorous for a time in Wardian cases ; 

 but this does not prove that their Creator made a 

 mistake when he placed vegetables in the open air. 



Plants confined in houses or other close structures, 

 may be made to grow in spite of such confinement ; 

 but all experience proves that other favourable cir- 

 cumstances, such as heat, light, and moisture, being- 

 equal, those plants are most vigorous and healthy 



