in Combustion, and in the Condensation of Vapours. 301 
€ans, or capable of acting in a sensible manner upon our’ 
apparatus, it is necessary that the quantity of the new sub- 
stance formed should be considerable; for it is certain that 
the most intense heat, if it be developed in a very small par- 
ticle of matter, may exist without producing any sensible 
effect which could give us any indications of its existence. 
It is not less true that the chemical union of two atoms, 
two different elementary substances, ought always, under 
every circumstance, to be accompanied with one and the same 
change of temperature : for'this union takes effect in a place 
so distant, relative to all the other bodies (if, in every case, 
all the interstices are not filled with particles of an ethere- 
ous fluid), that we cannot conceive how the change of tem- 
perature in question may be either augmented or diminished 
by the effect of the action of these surrounding bodies. 
It is extremely probable, from what we have been able 
to remark in a great number of phenomena, that the ap- 
proximation of the elementary particles of bodies is always 
accompanied by an elevation of their temperature; and as 
it cannot have new substances formed except in conse- 
quence of an approximation and the chemical union of 
elementary particles, we may conclude that there cannot be 
new chemical compositions without a development of heat. 
We may form an idea of what passes in combustion, by 
considering the phenomena which take place when water 
freezes. 
' Ata certain temperature, which is invariable, the mole- 
cules of the liquid are disposed to approximate in order to 
form a solid body, ice ; and the first particle of ice which is 
formed is accompanied by a development of a certain quan- 
tity of heat, which quantity is invariable. 
It is also very probable that it is at a temperature which 
is invariable, that the oxygen and hydrogen are disposed 
to approximate and unite in order to form an atom of va- 
pour, and that the intensity of the heat developed at the 
moment of this union is also invariable, and that it is al- 
ways manifested in all its intensity in the atom of vapour 
which is formed. 
‘ But as the atom of vapour is extremely small, and sur- 
rounded by bodies relatively very cold, its heat is soon dis- 
sipated. 
’ There is however a method, which appears certain, that 
we may employ in order to determine the temperature of 
an atom of vapour at the moment of its formation, and 
by this means we may know what is the highest tempera- 
' ture 
