XXVm INTRODUCTION. 



destruction in perpetual operation^ as well as the 

 law of renovation — were they not invariably linked 

 as it were hand in hand^ the surface of the earth 

 would become, in one extreme a desert, untenanted 

 by living things ; in the other, a self- destructive 

 crowd. 



Returning to the phial, and therefore to all closed 

 vessels or buildings, we cannot fail to perceive, that 

 while all the agents of life, all the vivifying principles 

 are allowed the fullest scope for their operations, all 

 the destructive ones are in a greater or less degree 

 excluded : Nature is still at work : no particle of the 

 benefit results from human skill : we add no gases to 

 those around us in order to make the air more nourish- 

 ing : we subtract none to make it more pure. Atmo- 

 spheric humidity is one of the most important agents 

 in the vitality and luxuriant 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 growing only 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 be exceeded in a state of nature. In 

 the rapid transitions from heat to cold, so common in 

 our climate, and so particularly injurious to tender 

 vegetables, these cases offer a complete barrier : for 

 experiments prove beyond question that the atmo- 

 sphere within the glass retains its degree of tempera- 

 ture very long after a change has taken place in the 



