184 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
element the meaning that it now possesses—the constituent of a 
compound. But in the middle of the seventeenth century chem- 
istry had not advanced far enough to make his definition useful; for 
he was unable to suggest any particular substance as elementary. 
And, indeed, the main tenet of the doctrine of phlogiston, promul- 
gated by Stahl in the eighteenth century, and widely accepted, was 
that all bodies capable of burning or of being converted into a calx, 
or earthy powder, did so in virtue of the escape of a subtle fluid 
from their pores; this fluid could be restored to the calces by heating 
them with other substances rich in phlogiston, such as charcoal, oil, 
flour, and the like. Stahl, however false his theory, had at least the 
merit of having constructed a reversible chemical equation: 
Metal — phlogiston=Calx; Calx + phlogiston = Metal. 
It is difficult to say when the first element was known to be an 
element. After Lavoisier’s overthrow of the phlogistic hypothesis, 
the part played by oxygen, then recently discovered by Priestley and 
Scheele, came prominently forward. Loss of phlogiston was identi- 
fied with oxidation; gain of phlogiston with loss of oxygen. The 
scheme of nomenclature (Méthode de Nomenclature chimique), 
published by Lavoisier in conjunction with Guyton de Morveau, 
Berthollet, and Fourcroy, created a system of chemistry out of a 
wilderness of isolated facts and descriptions. Shortly after, in 1789, 
Lavoisier published his Traité de Chimie, and in the preface the 
words occur: ‘“‘If we mean by elements the simple and indivisible 
molecules of which bodies consist, it is probable that we do not 
know them; if, on the other hand, we mean the last term in analysis, 
then every substance which we have not been able to decompose is 
for us an element; not that we can be certain that bodies which we 
regard as simple are not themselves composed of two or even a larger 
number of elements, but because these elements can never be sepa- 
rated, or rather because we have no means of separating them, they 
act, so far as we can judge, as elements, and we can not call them 
simple until experiment and observation shall have furnished a proof 
that they are so.” 
The close connection between crocus of Mars and metallic iron, 
the former named by Lavoisier oxyde de fer, and similar relations 
between metals and their oxides, made it likely that bodies which 
reacted as oxides in dissolving in acids and forming salts must also 
possess a metallic substratum. In October, 1807, Sir Humphry 
Davy proved the correctness of this view for soda and potash by his 
famous experiment of splitting these bodies by a powerful electric 
current into oxygen and hydrogen, on the one hand, and the metals 
sodium and potassium, on the other. Calcium, barium, strontium, 
and magnesium were added to the list as constituents of the oxides, 
