806 WARDIAN CASES. 
cularly the case with sulphurous acid and hydrochloric acid 
gases, even in small quantities ; but an atmosphere containing 
much ammonia, common coal gas, cyanogen, &c., also acts pre- 
judicially. 
The action of sulphurous and hydrochloric acid gases upon 
plants appears to resemble that of irritants upon animals ; thus 
they first exert a local action upon the extremities of the leaves, . 
and this influence is soon communicated to the deeper tissues, 
and if the plants be not removed into a purer air, they will 
perish ; but when such gases are not in great quantities, if the 
plants are speedily removed from their influence, they usually 
revive, the parts attacked being alone permanently injured. 
While the gases thus mentioned act as irritant poisons upon 
plants, sulphuretted hydrogen, carbon oxide, common coal gas, 
cyanogen, and others, seem to exert an influence upon them 
like that produced by narcotic poisons upon animals, for by their 
action a general injurious influence is produced on their vitality, 
and a drooping of the leaves, &c., takes place ; and, morever, 
when such is the case, no after removal into a purer air will 
cause them to revive. . 
As the above gases are constantly present in the air of large 
towns, and more especially in those where chemical processes 
ona large scale are going on, we have at once an explanation of 
the reason why plants submitted to such influences will not 
thrive. The air of an ordinary sitting room, and especially one 
where gas is burned, is also rendered more or less unsuitable to 
the healthy growth of plants, in consequence of the production 
of injurious gases as well as from the dryness of the atmosphere. 
Wardian Cases.—In order to protect plants from the injurious 
influences thus exerted upon them by the soot and impure air 
of large towns, the late N. B. Ward introduced the plan of 
growing them under closed glass cases which has been found to 
succeed so admirably. These cases consist essentially of a box or 
trough in which a suitable soil is placed ; in this the plants are 
put. and the whole is then covered by a closely fitting glass case. 
it is necessary, at first, to water the plants freely. When plants 
are grown under such circumstances, upon exposure to light and 
air, transpiration takes place from their leaves, as under ordinary 
conditions of growth ; the fluid thus transpired is, however, 
here condensed upon the surface of the glass case which encloses 
the plants, and ultimately returned to the soil. It is thus 
brought into contact again with the roots of the plants, to be 
again absorbed and exhaled by them; and these changes are 
continually repeated, so that the plants are always freely ex- 
posed to moisture, and do not require a further supply of water 
for a considerable period. Those plants, especially, which suc- 
ceed best in a damp atmosphere, as is commonly the case with 
Ferns, do exceedingly well in such cases. The important in- 
fluence which is exerted by the invention is, the protection of 
