[Sidenote: Hydrogen and chlorin combined]
Just as hydrogen burns in the air, so it burns in chlorin. The burning
of hydrogen in air or oxygen is, as we have seen, simply the combination
of hydrogen and oxygen, the product being water in the form of vapor,
and therefore invisible. When hydrogen burns in chlorin, the action
consists in the union of the two gases, the product being hydrochloric
acid, HCl, which forms clouds in the air. The two gases, hydrogen
and chlorin, may be mixed together and allowed to stand together
indefinitely in the dark, and no action will take place. If, however,
the mixture be put into a room lighted by the sun, but where the sun
does not shine directly upon it, combination takes place gradually;
but if the sun be allowed to shine directly upon the mixture for an
instant, explosion occurs, this being the result of the combination of
the two gases. The same result can be caused by applying a flame or
spark to the mixture. In this case light causes chemical action. The art
of photography depends upon the fact that light has the power to cause
chemical changes.
[Sidenote: Importance and preparation of hydrochloric acid]
I will here consider hydrochloric acid somewhat in detail, because it
is very important in the digestion of food, being the principal fluid
composing the gastric juice of the stomach. Hydrochloric acid is always
made by treating common salt (one afflicted with acid fermentation
should omit the use of salt and soda), under high temperature, with
sulfuric acid. This product is given off as a gas, which dissolved in
water forms hydrochloric acid, sodium sulfate remaining behind as a
result of this process. The chemist describes the action that takes
place by writing what is called a chemical equation, as follows: