HALES EXPERIMENTS. 303 



quantity of scoi-cliiug vapours in the nuddlo tlian outside of the 

 ground, and that being a denser medium, it was much hotter 

 than a more rare medium. 



' The gardeners about London have, to their cost, too often 

 had occasion to observe a similar effect, when they have incau- 

 tiously put bell-glasses over their cauliflowers early on a frosty 

 morning, before the dew was evaporated off them ; which dew 

 being raised by the sun's warmth, and confined within the glass, 

 did then form a dense transparent scalding vapour, which burnt 

 and killed the plants.' 



These observations translated into the language of the present 

 day clearly show how acutely and exactly Hales comprehended 

 the influence of perspiration upon the life of plants. 



According to him, the proper thriving of plants depends 

 upon the supply of food and moisture from the soil, which 

 again is governed in a measure by a certain temperature and 

 dryness of the atmosphere. The imbibing power of plants, — 

 the motion of the sap in them, — is dependent upon exhalation ; 

 the quantity of food imbibed and needed for the functions of the 

 plant, is proportionate to the quantity of moisture exhaled in a 

 given time. If the plant has imbibed a maximum of fluid, and 

 the exhalation is hindered by a low temperature, or by long- 

 continued wet weather, the supply of food, or the nutrition of 

 the plant stops, the sap stagnates, and an alteration ensues 

 tending to the generation of parasitical microscopic growths. 

 If rain falls after hot weather, followed by a strong heat without 

 wind, and every part of the plant is surrounded with an atmo- 

 sphere saturated with moisture, cooling by further exhalation 

 ceases, and the plants succumb to the sun-blasts. 



APPENDIX D (page 91). 



ANALYSES OF DRAINAGE, LYSIMETEU, RIVEK, AND MARSH WATER. 



I. — Drainage Water. 



Thomas Way found in drainage water taken from seven dif- 

 ferent fields, the following constituents (' Journal of the Ivoy. 

 Agric. Soc.,'vol. xvii. 133): — 



