THE NATURAL CONDITIONS OF PLANTS. 27 
in the atmosphere of London such a proportion of 
noxious gases as sensibly to affect vegetation, since 
we find geraniums and many other plants growing 
well without crisping or curling of their leaves, 
in the windows of shops and small houses, pro- 
vided care be taken to keep the plants clean and 
free from soot. In the closed cases direct con- 
tact with any current of noxious gases is pre- 
vented, and the action of the law which regulates 
the diffusion of gases prevents the admission of 
such quantities of any noxious gas as might be 
injurious to the plants. 
As this is a subject of the highest importance 
to the well-being of everything that has life, 
whether vegetable or animal, it will be well 
to give a full explanation of the above men- 
tioned diffusion law — a law constantly in opera- 
tion under all circumstances, and without the 
beneficent operation of which vegetable as well as 
animal life would suffer greatly in large towns, 
and a cellar in St. Giles’ would t quickly become 
a grotto del cane. 
“If we take two vessels, and fill one with car- 
bonic acid gas, and the other with hydrogen 
(their weights respectively being as twenty-two to 
one), and then place the light gas perpendicularly 
over the other, effecting a communication between 
the vessels by means of a tube not larger in 
