XXVlll 
INTRODUCTION. 
agents in the vitality and luxuiiant growth of ferns ; and 
this is attained in closed cases, or under bell-glasses, in 
such perfection, that the most moisture-loving of all our 
species — Trichomanes speciosum, of which I have before 
spoken as delighting in the spray of water-falls — not 
only lives but thrives. Mr. Ward has this plant growing 
with a luxuriance and vigour that can seldom he exceeded 
in a state of nature. To the rapid transitions from heat to 
cold, so common in our climate, and so particularly injuri- 
ous to tender vegetables, these cases offer a complete har- 
rier 3 for experiments prove, beyond question, that the 
atmosphere within the glass retains its degree of tempera- 
ture very long after a change has taken place in the air 
that surrounds it, and excess of cold, accompanied by per- 
fect stillness, is incomparably less injurious tlian when 
coupled with rapid motion. Thus our travellers in Polar 
regions speak of intense cold, as indicated by the thermo- 
meter, having been scarcely inconvenient to them if the 
atmosphere were perfectly still ; but if the wind rose, al- 
though the quicksilver rose simultaneously, as was almost 
invariably the case, the cold was most distressing. In 
England, if Fahrenheit’s thermometer he at 30'’, we walk 
about or stand exposed to it without any sensation of pain, 
but if we face it in travelling by railway at the rate of 
thirty miles an hour, the cold becomes almost intolerable. 
In fact, it has been abundantly proved by experiment, that 
a much greater extreme of heat or cold may he borne by 
plants, by animals, and even by the human frame, if both 
the atmosphere and the objects of experiments be in a 
