496 Scientific Proceedings, Royal Dublin Society. 
heat due to distinctly physical change is divisible into five 
quantities ; viz., the heat capacity or specific heat of the body in 
the solid, the liquid, and the gaseous states, and the latent heat of 
fusion and of vaporization. ‘The heat due to distinctly chemical 
change is divisible into two quantities, the heat of formation 
from the elements, and the heat of allotropic or molecular modi- 
fication. 'To these must be added the heat of hydration, solution, 
and dilution, in aqueous and other solvents, changes of a partly 
physical and partly chemical character. ‘That is for each individual 
substance taking part in a reaction, we have no less than ten 
distinct quantities to keep a record of, so that with a very simple 
chemical equation, involving no more than two reagents and two 
products, we might have no less than forty magnitudes to deal 
with. It is therefore highly desirable that we should be able to 
briefly and clearly indicate them, without burden to the memory, 
without confusion in the calculation, and without destroying the 
purely chemical interpretation of the equation. 
Inasmuch as matter and force are indestructible, if we start 
with any given quantity of matter and force, and go through any 
number of operations, we must still have the same quantity of 
matter and force, and to keep a clear and intelligible record all 
that is necessary is to express precisely the total matter and energy 
at the initial and final stages of our reaction, and see that those 
balance. The energy that invariably falls to be measured is that 
evolved by our operation. If we were able to state exactly the 
total energy associated with each particular element or compound 
in any reaction, and coupled such with the symbol expressing the 
matter thereof, then the energy evolved in a reaction would simply 
be added to the equation as one of the products of the reaction, 
after the manner of an ordinary chemical equation, and the equa- 
tion would then balance both in matter and energy ; but there is 
no hope that we will ever be able to ascertain the total absolute 
quantity of energy associated with any substance, for no matter 
what processes we subject it to, we can never be certain that we 
have depleted it of all its energy. The only energy of which we 
have any real knowledge is that evolved in the transition of any 
system of atoms or molecules from one state to another. It is 
therefore necessary to select arbitrarily a particular state as our 
standard of reference and then indicate the heat evolved, be it 
