i8o 
On Water and Air. 
[March, 
I will now take another jar containing a bundle of atoms 
of another gas (if I may use the term). This gas is 
hydrogen. In the case of hydrogen, I take the jar which is 
to receive the gas and hold it upside down ; because the 
gas, being very much lighter than air, will rise into the jar, 
like a light body in water, and rapidly fill it. 
The gas is now entering the jar, and I trust that we shall 
displace the air in this manner. [The jar having been filled, 
the hydrogen was ignited.] If I apply a light you will see 
that I have gathered a combustible gas which burns with 
the flame which you see before you. If you were close to 
this jar you would see that it is covered with moisture. 
What was really done when this flame was produced ? In 
point of fadt, the hydrogen burnt in the oxygen of the air. 
And what was the result of the union of the oxygen with the 
hydrogen ? The result of their union was the production of 
our familiar substance, water ; so that water is composed of 
these two substances — oxygen and hydrogen, combined 
together in definite proportions, as chemists say. The 
atoms of hydrogen and the atoms of oxygen coalesce, and 
they form little groups of atoms. And here I shall have to 
use another term of which you ought to know the meaning. 
The manner in which these things combine is this : two 
hydrogen atoms single out one oxygen atom, and unite with 
it, so that the smallest particle of water is composed of a 
group of three atoms, two of which are hydrogen and one of 
which is oxygen. This group of atoms has a different name 
from the atoms themselves. It is called a “molecule.” 
Hence we have two “ atoms ” of hydrogen and one “ atom ” 
of oxygen, and we have one “ molecule ” of water produced 
by their union. We can take water and produce from it 
these gases very easily indeed. The ordinary way of producing 
this hydrogen gas is to pour a little dilute sulphuric acid 
upon zinc. Here are fragments of zinc, and here is sulphuric 
acid diluted with water. The zinc has, as chemists say, a 
power of attraction which it exerts upon oxygen, and when 
this liquid is poured upon zinc, the zinc seizes upon the 
oxggen and allows the hydrogen to go free. I will now pour 
into this globular vessel, containing fragments of zinc 
(Fig. 16), some dilute sulphuric acid, and you see we get a 
stream of hydrogen gas issuing from the liquid. 
By applying a light to the end of the jet attached to the 
apparatus the issuing gas can be ignited, and it burns with 
a pale, lambent flame. 
In this case we use granulated zinc ; that is, zinc broken 
up into small pieces, for the purpose of enabling the acid 
