302 CHEMICAL COMBINATIONS 



the horses have been fed, with the compounds of salts and ani- 

 mal matter, all of which contain within themselves a tendency 

 to rapid putrefaction, and necessarily evolve a large amount of 

 ammonia. This is the principal source which gardeners have 

 had to draw upon for a supply of this agent ; and, although 

 exercising the most striking effects, it is rather remarkable that 

 the cause of these effects should, until lately, remain a mystery 

 to gardeners in general, and that the same elements, in a more 

 concentrated state, should not, in other circumstances, be applied 

 to produce the same results. 



The second question is, perhaps, of more difficult solution. 

 Plants are living, organized, beings, and acted upon, atmospher- 

 ically, chiefly by the glands that cover the surface of the leaves ; 

 and abundant evidence exists, that they are as susceptible of 

 either injury or benefit, through the medium of the atmosphere 

 to which they are exposed, as animal life, and our ignorance of 

 the effect of houses artificially heated, upon the delicate organ- 

 ism of plants, is only accounted for from the fact, that com- 

 paratively little attention has yet been paid to this branch of 

 horticultural science by practical gardeners, and still less has it 

 been applied to the culture of exotic plants. If, for instance, 

 we take a plant from the open ground, where it is fully exposed 

 to the pure air, plant it in a pot, and place it in a close living 

 room, or in a hot-house, the effect will be rendered obvious by 

 the altered appearance of the plant. Again, if we take a plant 

 newly potted, and otherwise disturbed in the roots, and set it in 

 an arid situation, and fully exposed to the air, the leaves will 

 be withered and dried up in a few hours, and probably the death 

 of the plant will be the issue. But if the plants are placed in 

 a close, moist atmosphere, the results will be very different. 

 Now these illustrations are common, and, in themselves, exceed- 

 ingly simple, so much so, that we frequently observe them, and, 

 if asked the cause, we give a kind of generalizing reply, by 

 attributing it to the sun, or some such cause, which is well 

 known to be the principal origin of heat, yet they serve to show 

 how susceptible plants are of influences which, strictly speak- 

 ing, are neither dependent upon heat nor cold, although these 

 two latter elements are almost the only ones which .ve are in 



