i88o.l The Evolution of Scientific Knowledge. 425 
to be out of harmony with the facets, and was therefore 
worsted in the struggle for the Survival of the Fittest. . It 
was found that chemical substances during combustion, 
instead of losing weight by the abstraction of phlogiston, 
gained in weight, the products of combustion being heavier 
than the body burnt. It was in vain to try and “ bolster up 
the old theory ” by the hypothesis that phlogiston is so light 
in its nature that it makes the bodies with which it combines 
lighter than they were before, though this view seems 
seriously to have been entertained. When Lavoisier found 
that mercury, on being heated for a long time in the air, 
absorbed oxygen and increased in weight, and that when the 
material formed was subsequently heated it lost weight and 
gave up just so much oxygen as it had before absorbed,, the 
Phlogiston theory was doomed, and the science of chemistry 
was placed on its true basis ; while at the same time, and 
by the same man, the foundation of quantitative chemistry 
was laid by the distinct assertion of the indestructibility of 
matter. 
But we cannot say that a true quantitative theory of the 
chemical constitution of matter was in existence before 
Dalton, in 1803, published his first table of atomic weights, 
and showed “that elements combine in definite proportions; 
that these determining proportions operate reciprocally ; and 
that when, between the same elements, several combining 
proportions occur, they are related as multiples.” This 
conception of definite combination by weighty which has 
been termed “ the pole star round which all other chemical 
phenomena revolve,” was not long afterwards supplemented 
by Gay-Lussac’s law of definite combination by volume , 
which received an explanation in 1811 at the hands of Avo- 
gadro, who assumed that equal volumes of all substances, 
when in the state of gas and under like conditions, contain 
the same number of molecules, — an hypothesis which is 
borne out independently by both chemical and physical con- 
siderations. 
When we add to the advances in chemical science already 
noticed Sir Humphry Davy’s discovery of the compound 
nature of the alkalies ; the determination, by Dulong and 
Petit, that the atoms of many of the elements have the 
same capacity for heat ; the proof, by Berzelius, that organic 
bodies obey the same laws as inorganic substances ; followed 
by the artificial production, by Wohler, of urea, a material 
before supposed to be exclusively the product of life : when 
we consider the overthrow of the dualistic theory, that salts 
result from the union of an acid oxide with a basic oxide, 
