The injury done to the tender foliage of plants in forcing houses, by contact with cold air, results from the increased capacity of air for moisture, as it become heated. When cold air is admitted to these structures, it cannot contain so great a quantity of aqueous matter, as it is capable of taking up when it becomes warmed: this increase of temperature, is soon in great measure, supplied to it, but rarely is a sufficient quantity of moisture, at the same time within its reach, to enable it to supply its increased capacity for aqueous matter: the consequence is, that on coming in contact with the foilage of the plants, which is of a succulent nature, and contains a great proportion of water, the warmed air continues to abstract a portion of moisture from the plants, until its capacity is satisfied; and hence the plants are robbed of their "life's blood." Besides this action, which is the cause of serious evil, the tissue itself is contracted and thereby injured, by reason of the degree of cold, which is at the first gush, liable to come in contact with the warm foliage. These remarks apply to cold air, when admitted in a large bulk, by opening the sashes; and when a draught is produced, by opening them, both at the back and front, and the top and bottom of the house. Deterioration of the air, by the action of the functions of the plants, could not take place, except in hermetically sealed structures: for by reason of the expansibility and elasticity of air, when it becomes at all heated, it not only gains egress, but also admission through the most minute crevices: that this interchange is sufficient to counteract any deteriorating influence which the plants might have on the internal air, with respect to their continued existence in it, is abundantly proved by the growth of plants in Ward's cases, from the interior of which the external air is excluded as fully as it possibly can be, without their being actually sealed: if therefore, any injurious effects result to plants, from their being cultivated in a close atmosphere, we must seek for the cause, in some other source, than the plants themselves. If any noxious qualities exist in the atmosphere of structures, to which the external air has not free ingress, they must result from some neglect or ignorance on our part, in suffering extraneous and unwholesome matters to accumulate in such situations, and there to decompose, and enter into combination with those gaseous bodies, which form the volume of the internal atmosphere of our plant structures. The existence of such extraneous matters, may indeed be traced to various sources; and they may be present, even when much vigilance is employed to prevent their accumulation; and therefore, as an inconceivably minute quantity, inappreciable to the senses, would frequently be sufficient to effect deterioration, it is possible that these impurities may often originate in sources which are least of all suspected. The decomposition of organic