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of Edinburgh, Session 1864-65. 
According to Stahl, all combustibles contain one and the same 
substance in different proportions, according to the degree of their 
combustibility. That substance is phlogiston ; and when a com- 
bustible is burnt, or a metal calcined, its phlogiston is given out. 
When charcoal or oil is heated with a metallic calx, the phlogiston 
leaves the former, and is found in combination in the metallic 
regulus. 
Now, if we consider the facts of the case in an unprejudiced way, 
we must admit that a combustible loses something when it is burnt, 
it loses combustibility, or the capability of being burnt. In the 
same way, in the preparation of phosphorus, or the reduction of a 
metallic calx, the charcoal loses this capability, while the phos- 
phorus or metal acquires it. 
The capability of being burnt is essentially the power of emitting 
a certain quantity of heat, and, as we know from the researches of 
Rnmford, Davy, and others, and from the later and more accurate 
determinations of Joule, that heat is a particular form of what has 
been called kinetic energy, we can have no difficulty in admitting 
that the power of emitting a certain quantity of heat is a particular 
form of potential energy. 
If, in the statement of the phlogistic theory, we read potential 
energy for phlogiston, and understand that when phlogiston is 
separated from one body and not taken up by another, as in combus- 
tion, this potential energy is converted into kinetic energy, we have 
a tolerably complete account of what we now know of the matter. 
Whether we choose, with Beecher and Stahl, to call phlogiston a 
substance or not, depends on our definition of what a substance is, 
If we restrict that name to ponderable matter, of course it is not a 
substance, but when we consider’that energy is as indestructible as 
matter, that we can trace it through its various combinations and 
double decompositions, and that we are in a fair way to discover, 
not, indeed, its atomic weight, for it has none, but its chemical unit, 
it does not seem very absurd or unreasonable to class it along with 
the ordinary chemical elements. 
It may be objected to the phlogistic theory, as thus explained, 
that it is not the combustible alone, but the combustible and oxygen, 
that have potential energy, and that it is only when the two unite 
that this potential energy is transformed into kinetic. This objec- 
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