670 Hygiene of House Plants. [ November, 
decreases. So long as nitrogenous matter is in excess carbonic 
acid may be given off, but when the carbonaceous matter is in 
excess, whatever carbonic acid is set free is at once taken up by 
the chlorophyl and the carbon fixed, the oxygen being set free ; 
and this latter is the great work of plants. So great are its ef- 
fects that it is believed that they wholly counteract the vitiating 
influence of the billion pounds of carbonic acid which are, as is - 
estimated, annually sent into the atmosphere ; and throughout 
all the geological ages, since the development of plant life in its 
higher forms, it has been taking carbonic acid from the air, fix- 
ing the carbon and restoring the oxygen. Every pound of coal 
in all the two hundred thousand square miles of coal area in 
North America, represents three and two thirds pounds of car- 
bonic acid taken from the air. 
It can make but little difference where plants grow; those 
conditions which are essential to their growth must be met. If 
forests purify the air about them, it is reasonable to suppose 
that smaller groups of vegetation in our houses will purify that 
about them. There are indeed some plants that revel in filth 
and noisome vapors, but they are not such as will. be found in 
our houses. Modern plants are many of them unable to endure 
even a slight increase in the amount of carbonic acid in the sur- 
rounding air, and we are forced to suppose that the plants of the 
coal period were peculiarly fitted for the atmosphere in which 
they grew. It has been found that many gaseous and other sub- 
stances affect animals and plants in a similar manner, and m 
many cases an atmosphere in which one will not thrive is hurtful 
to the other. Many injurious gases that are too often found in 
our dwellings affect plants even more readily than they do me 
so that to a certain extent plants become tests of the air we 
breathe ; and when it is found that plants will not grow m s 
room because of gas from chandelier or furnace, it is surely true 
that such rooms are unfit for man’s occupation, and that they can- 
not be used without certain injury to the health. In greenhouses, 
where a large number of plants are shut up in a small amount of 
air, it is true that the amount of carbonic acid is, even at night, 
less than outside. Florists, who spend much of their time 1P 
- greenhouses, are as a class unusually healthy, and sometimes 
with not the 
Rag eet Ras ROM rR Nee Le ie Oe” aah aie rane ee 
