VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 221 



manured, as the body of earth in which they grow. It may 

 appear vague and visionary to talk of manuring the atmosphere 

 of a hot-house, but the thing is in reality neither so vague nor 

 yet so visionary as it seems ; for here science comes to our 

 aid, and not only defines the vagueness, but converts the vision 

 into a practical reality. It proves to us, both the benefits of 

 manuring the air, and the manner of doing it. We know that 

 plants derive a large portion of their food from the atmosphere ; 

 and we know, also, that the arid atmosphere of a hot-house is not 

 always charged to a proper degree with these life-giving gases. 

 An impoverished atmosphere must have the same effect as an 

 impoverished soil. This is a well-known fact, and requires no 

 demonstration to prove it. We are well aware that many 

 plants will grow luxuriantly for years, suspended in the air, pro- 

 viding they be kept in a condition calculated to sustain them; 

 but deprive them of these gases, and they will die, — deprive 

 the atmosphere of its humidity, and they will quickly cease to 

 exist as living plants. These vegetables absorb carbonic acid, 

 ammonia, and water, from the atmosphere, by their leaves, even 

 more abundantly than by their roots. This is especially the 

 case with plants cultivated in pots ; their roots being circum- 

 scribed into a small space, the nourishment is speedily exhausted, 

 and if the atmosphere be at the same time robbed of its gaseous 

 elements by artificial heat, the plants must perish, if this defi- 

 ciency is not supplied to them by artificial means. 



We have seen, that plants, even of a ligneous nature, will 

 grow, form lignin, and proteine compounds, while suspended in 

 a moist warm atmosphere, much in the same manner as plants 

 do when growing in the soil. The amount of mineral matter 

 they contain is indeed very small, and may be derived from the 

 dust continually floating in the atmosphere, which is dissolved 

 as it falls upon the leaves, and is absorbed with the atmospheric 

 fluids. Here, then, we have plants subsisting upon the ingre- 

 dients of the atmosphere ; and experiments seem to prove that 

 all plants are nourished by the same substances, in variable 

 proportions, the chief of which are carbonic acid, water, and 

 ammonia. 



Experience has already proved the beneficial effects of these 

 substances as fertilizers, not only of the soil, but also of the 



