OF THE ATMOSPHERE OF HOT-HOUSES. 303 



the habit of supplying to our plants by measure, and that, too, 

 in the most unnatural proportions, while the ammoniacal and 

 hygrometrical condition of the atmosphere is generally left to 

 uncontrolled transmutations of chance. 



5. It may be asked, " What guide have we to ascertain the 

 condition of the atmospheric gases ? " 



In the present state of our knowledge of gaseous bodies, their 

 presence or preponderance in the atmosphere of hot-houses must 

 be little else than a matter of conjecture. An experienced gar- 

 dener, on entering his hot-house in the dark, can tell pretty 

 accurately what degree of temperature the atmosphere of the 

 house is standing at, by the sensation produced upon his face, 

 or by the wave of his hand in the air. Now, in regard to the 

 excess of volatile gases floating in the atmosphere, the organs of 

 smell are much more delicate indicators than the sense of feeling. 

 This is more especially the case when the house is close, and the 

 temperature pretty high; for then the ammonia, being little more 

 than half the weight of the common atmosphere, [more nearly 

 three fifths, its specific gravity being 0.59, that of air being 1 J 

 hence, when liberated on the floor, or on the flue, pipes, tank, or 

 other heating apparatus, it readily rises and mingles with the 

 atmosphere ; and although it requires a considerable proportion 

 of it in the atmosphere to be injurious, or even offensive to the 

 senses, it is, nevertheless, easily detected by those acquainted 

 with this gas, even when present in small quantities, and the 

 experienced organs of the practical man have no difficulty in 

 deciding whether or not it is present in excess. On entering a 

 hot-house, when oxygen arid aqueous vapor are deficient in the 

 atmosphere, this fact is at once detected by the oppressive burnt 

 smell which pervades the house. Saturate the atmosphere with 

 water, oxygen is generated, and the smell ceases. The carbonic 

 acid, which previously existed in excess, combines with the oxy- 

 gen, and is transformed into carbonic acid gas, in which state it 

 is assimilated by the plants. In the state of vapor, water exer- 

 cises a wonderful influence over the atmosphere of a hot-house, 

 and ministers most materially to the life and growth of plants. 

 It is in the form of water, indeed, that nature introduces the 

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