12 
MANUKES. 
gen gas. Now this word gas is a chemical term, and 
it means any substance in vapor, which cannot be 
condensed into a liquid or solid, at common tempera¬ 
tures. Different gases may unite, and so become 
solids or liquids. Steam is not gas, for it is the vapor 
of water, and immediately returns to the state of 
water, below 212°. Perfect steam is invisible, so are 
most gases. The air we breathe is composed of two 
gases, oxygen and nitrogen. We do not see them ; 
we cannot, by cooling or compression, make air take 
other shape than invisible air. This is the general 
property of gas, as distinguishod from vapor or steam. 
Oxygen and hydrogen, in plants, exist in just the 
proportions to form water, but we do not know that 
they are united in these proportions. We have com¬ 
pelled them to unite, by heating the substance or 
root. The carbon is by this same process consumed, 
and, you know, has thus formed carbonic acid. Be¬ 
sides this, a portion of the carbon unites with some of 
the hydrogen of the plant. This forms light, inflam¬ 
mable air. Now you may collect this light, inflam¬ 
mable air, in any stagnant water where plants are 
decaying. Decay gives exactly the same products as 
are formed in making charcoal. Decay is only slow 
combustion, or burning; no matter whether we char 
the plant or leave it to decay, we obtain exactly the 
same products as we did by our analysis, that is, 
carbon and salts. 
MOULD. 
But because there is not heat enough, we leave by 
decay a portion of the hydrogen and oxygen still 
united to the coal. A slow mouldering fire leaves pro¬ 
ducts more like those of decay. Decay is a slow, 
mouldering fire; hence the products of the decay of 
plants are very aptly termed mould . It is the pro¬ 
duct of a mouldering fire; that is, an imperceptible 
union of the oxygen of the air with the carbon of the 
