ARTIFICIAL ILLUMINATION. 155 
covered. I have no fear that the gas will escape, as it is so much 
heavier than the atmosphere. You see that when I put this lighted 
taper into it the light is extinguished immediately, and the carbonic 
acid gas does not take fire. This gas then will not support com- 
bustion, and therefore it will not support life, for one of the features 
of animal life, you know, is the combustion of the carbon of the food 
in the veins and capillaries of the living body by the oxygen absorbed 
through the lungs. When we breathe we take in oxygen which, being 
absorbed by the blood, oxidises the carbon of the food which has 
been assimilated. The carbonic acid thus generated, and which is the 
combustion of the carbon of the food, in the veins and capillaries of the 
living body, by the oxygen absorbed through the lungs, the cause of the 
dark colour of the blood in the veins, is got rid of at the lungs at 
each expiration of the breath. This gradual oxidation, or slow 
burning of the carbon in our food, is the cause of the gentle warmth 
of our blood. The carbonic acid gas, which is thus being expelled 
from the lungs of every living animal, and which is also generated 
in huge volumes in every active fire-place and furnace in the world, 
and is also a product of fermentation and many other natural and 
artificial processes constantly going on, would in time so pervade 
the atmosphere that it would be impossible for any animal to exist in 
it. A wise provision of nature, however, remedies all this. Under 
the influence of the direct rays of the sun, or diffused daylight, the 
leaves of plants decompose the carbonic acid gas existing in the atmos- 
phere, absorbing to themselves the carbon with which they build up 
their stems and woody fibres, and restoring to the air the pure oxygen 
which is so necessary for animal life. 
As a further illustration of the fact that nothing is really destroyed 
when combustion takes place, I may mention that when a ton of coals 
is burnt in a fire-place nearly four tons of carbonic acid go up the 
chimney ; thus neither the coals nor the oxygen with which it has 
‘combined has been really lost; they are in a combined form instead 
of having a separate existence. 
Now the chief substances which are burnt to produce light are 
oil, petroleum, tallow, wax, coal gas, etc. These are all hydro- 
carbons, that is to say, they consist of hydrogen and carbon combined 
in various proportions, and, therefore, when they burn in air, the pro- 
ducts of the combustion are necessarily always water and carbonic acid. 
I should qualify the statement by saying that the products of the perfect 
combustion of these substances are water and carbonic acid. When 
carbon, or any hydro-carbon, is burnt in an insufficient supply of air 
the carbon may unite with only half the proper amount of oxygen, and 
the gas known as carbonic oxide may thus be formed, which is far 
more deadly than carbonic acid, inasmuch as it actually poisons the 
blood of those who inhale it, while carbonic acid simply stifles or as- 
phixiates through its presence diminishing the amount of oxygen in the 
air which is necessary for healthy breathing. 
In addition to the deleterious carbonic acid, which is one of the pro- 
ducts of the combustion of coal gas and other hydro-carbons, the water, 
which is the other product, is often troublesome in another way. For 
