FUNCTION OF CRITICISM IN ADVANCEMENT OF SCIENCE. 355 
development of our ideas of energy, from the theory of phlo¬ 
giston to the laws of intensity and capacity, is one of the 
most fascinating and instructive accessible to the student of 
science. The phlogistic theory, invented by Stahl at the 
end of the seventeenth century, was in vogue for about one 
hundred years, till Lavoisier overthrew it finally. It main¬ 
tained the notion that every combustible substance contained 
an inherent principle which it lost during combustion. 
Hence all combustible substances are compounds. Charcoal, 
coal, sugar, flour, &c., are rich in phlogiston. The actual 
escape of flame from a burning substance as a visible some¬ 
thing, the uncertainty of the nature of heat, and the idea 
that it is a form of imponderable matter, caloric, gave the 
theory its strength. Besides, no other theory was known 
which could take its place. Oxygen was dephlogisticated 
air because it was capable of taking more phlogiston from 
nitrous air, and therefore must itself contain less of the prin¬ 
ciple. Nitrogen was phlogisticated air because it could give 
up the burning material. 
This theory fell under the blow which Lavoisier dealt it 
with his balances. He weighed his gas and found that what 
one lost on entering into chemical combination, the other 
gained in weight. He found that air consisted of a mixture 
of two gases, one of which could support combustion, while 
the other could not. He proved that water was a compound 
of hydrogen and oxygen, uniting in definite proportions. 
His opponents, the adherents of the phlogiston theory, 
Priestly , Cavendish, did their best to break the force of these 
experiments; but they never succeeded, and the theory per¬ 
ished with them, so far as phlogiston is concerned. 
The similar theory of caloric had a life for nearly 50 years 
longer, till about 1845, when Joule by experiment proved the 
mechanical equivalence of heat and work ; in other words, 
that heat, no more than the principle of combustion, is a 
substance. Having arrived at this negative proposition, it 
is not too much to say that after 50 years more of eager study 
we do not yet know what heat is, although we have discovered 
