1901-2.] Dr Marshall on Modifications of Sign of Eguality. 87 
(b) For reversible actions such as dissociations, etc., the sign 
thus : — 
CaC0 3 ^Ca0 + C0 2 ; 
2H 2 0^2H 2 + 0 2 ; 
H 2 S0 4 ^ IT + HSO; ^ 2H* + SO/. 
(c) For reversible actions associated with a definite transition 
temperature, the sign above which might be noted the 
appropriate temperature,* thus : — 
Na 2 Mg(S0 4 ) 2 ,4H 2 0 + 13H 2 0 Na 2 S0 4 , 10H,O + MgS0 4 , 7H 2 0 ; 
CuC1 2 ,2H 2 0 + 2K 2 CuC1 4 ,2H 2 0 
2KCuC 1 8 + Ii,CuCl 4 ,2H 2 0 + 4H 2 0 
3KCuC1 3 + KC1 + 6H 2 0. 
( d ) In cases where reversible actions go practically completely 
in one direction under ordinary conditions, and the reverse action 
may be rendered negligible, or when there is any doubt as to the 
reversibility of the action, and it is not desired to make a definite 
indication either way, the sign === might be used. This would 
apply to many of the equations representing the actions involved 
in analytical work, thus : — 
CuCl 2 + H 2 S — CuS + 2HC1; 
BaCl 2 + K 2 S0 4 =^= BaS0 4 + 2KC1. 
The symbols printed above f may be considered unduly large for 
the type employed ; they might easily be made considerably 
smaller if a still neater form is desired. 
It is possible that the general use of separate symbols to 
distinguish between equations representing mere equalities in 
quantities of matter, and those representing actual chemical 
changes, might go far to remind students of chemistry that a 
particular chemical change is not necessarily possible merely 
because an equation representing it can be produced on paper. 
* In such cases the barbs on the upper half of the symbol might be re- 
moved, which would simplify it somewhat without affecting its intelligibility, 
thus, 
t From type supplied by Messrs Miller & Richard, letterfounders, 
Edinburgh. 
(. Issued separately March 31, 1902.) 
