152 APPLICATION OF PRINCIPLES. 



during the time that the leaves are tender, and the 

 fruit unformed. Some excellent stoves have no pro- 

 vision at all for ventilation ; and we have the direct 

 testimony of Mr. Knight as to the disadvantage of the 

 practice in many cases to which it has been commonly 

 applied. 



It may be objected, says this great horticulturist, 

 that plants do not thrive, and that the skins of grapes 

 are thick, and other fruits without flavour, in crowd- 

 ed forcing-houses : but in these it is probably light, 

 rather than a more rapid change of air, that is want- 

 ing ; for, in a forcing-house which I have long de- 

 voted almost exclusively to experiments, I employ 

 very little fire heat, and never give air till my grapes 

 are nearly ripe, in the hottest and brightest weather, 

 further than is just necessary to prevent the leaves 

 being destroyed by excess of heat. Yet this mode of 

 treatment does not at all lessen the flavour of the 

 fruit, nor render the skins of the grapes thick ; on 

 the contrary, their skins are always most remarkably 

 thin, and very similar to those of grapes which have 

 ripened in the open air. (Hbrt. Trans., ii. 225.) 



While, however, the natural atmosphere cannot be 

 supposed to require changing in order to adapt it to 

 the respiration of plants, it is to be borne in mind that 

 the air of houses artificially heajed may be rendered 

 impure by the means employed to produce heat. 

 Sulphurous acid gas escapes from brick flues, am- 

 moniacal vapour from fermenting manure, and there 

 may be many unsuspected sources of the introduction 

 of vaporous impurities; an inconceivably minute 



