INTRODUCTION. 19 
the convenience of its form and the elegance of its 
appearance; indeed for transmuting the gardener’s 
_-hand-glass into the Wardian Case. I believe it to 
_ bea very great mistake to suppose that these truly, 
ornamental little gardens possess any other advan- 
a tages than those I have described ; that they exclude 
sulphurous acid gas, as suggested by Professors 
. Turner and Christison, or hydrochloric and muriatic 
2 acid gases, as supposed by others, must certainly be 
an error: we add no gases to those which exter- 
nally surround the case, and we keep none away: 
we cannot exclude them. With ferns atmospheric 
humidity is one great desideratum ; perfect stillness 
is another; equal temperature, avoiding extremes or 
sudden changes, is a third; the exclusion of fuli- 
_ ginous matter, and indeed of all dust and im- 
purities, is a fourth; and the modification of solar 
| : 
rays, which should never enter your fern-house, is — 
a fifth ; thus the injurious power of natural agents 
is restrained and restricted, whilst the beneficial 
: power of natural agents is retained and cherished, 
and its efficacy ensured: such is the effect of 
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