Pottok—A New Thermo-Chemical Notation. 497 
positive or negative, in the transition from this state to any other. 
The particular state that I believe to be the most convenient for 
the standard of reference is that of the elements in their normal 
condition at 0° C., but by common consent 15° C. has already been 
chosen. The heat of formation of a gram molecule of any parti- 
cular compound is therefore the energy evolved, and measured as 
heat in its formation from the elements in their normal condition 
at 15°C. This evolved energy I indicate by affixing to the 
formula the letter € with the numerical value of the heat evolved, 
represented in kilogram or gram degrees per gram molecule, and 
indicated by the letters H° and k° as suggested by Mr. J. Y. 
Buchanan, thus €H,O = 69K° = 69000kK° represents that the heat 
evolved in the formation of water is equal to 69 kilogram degrees 
or 69,000 gram degrees per gram molecule. Where a substance 
undergoes change of state, and it is desired to indicate the heat 
evolved in this change, without reference to that of the formation 
of the molecule from its elements, this is done by enclosing the 
formula in brackets and indicating the initial and final states, at 
the bottom and top of the evolved energy sign. For example, 
if x be the heat evolved in the transition of water from the state 
‘<q’ to the state ** b,” this would be indicated thus :— 
E(H.0) = «K°. 
All quantities must be so dealt with that 
€(H.0) + &(H.0) + é°(H.0) = €"(H.0), 
and 
&(HL.0) = ~ &'(H,0). 
If, on the other hand, we wish to indicate the heat evolved when a 
system undergoes chemical change, the state of all the substances 
reacting is represented by the ordinary chemical notation, the 
initial state of the system being indicated at the lower part of the 
evolved energy sign, and the products of the reaction, or of the 
final state, at the upper part. Thus the formation of water from 
H20 
H2+ 0? 
cases the lower part is understood and the symbol contracted to 
the form already shown €H,O and the complicated cases will be 
dealt with later under equations. To indicate the physical state 
its elements might be represented by &€ but in all simple 
