92 
SIR B. C. BRODIE ON THE CALCULUS OF CHEMICAL OPERATIONS. 
and a unit of sulphide of mercury into a unit of mercury and a unit of sulphide of 
hydrogen; thus 
§0-|-a=a0-j-5, 
(i-a)(0-l) = O. 
In which event “ no weight ” is constant, and the event occurs either by the “ substi 
tution” of a for S or by the transference of 0. 
Example : — Or, again, the transformation of a unit of bin oxide of hydrogen into a 
unit of hydrogen and a unit of oxygen, which may be thus expressed, 
(«-l)((£)-l)=0. 
In which event, again, “no weight” is constant, and the event occurs either by the 
transference of a or the transference of £ 2 , as is apparent from the equation 
i+«(£ 2 )= a +(£ 2 )- 
(10) The consideration of an event, not as a “ substantive ” event, but as “ the reverse ” 
of some other event, is a principle of very real utility, both as regards our appreciation 
of the real “ causes ” of events, and also as regards our appreciation of the analogies 
and relations of events. 
Take, for example, the event of the transformation of a unit of binoxide of hydrogen 
and a unit of sulphurous acid into a unit of sulphuric acid, the result of which is 
given in the equation 
a £ 2 + ^ 2 =l+^| 4 . 
Now this equation may be expressed by the prime factors (a| 2 ) and (0| 2 ) and written 
thus, 
(«r)+(^)=i 
The symbol of this occurrence, regarded as a substantive event, is 
(1 — a| 2 )($f 2 — 1 ) = 0, 
which compels us to interpret the factor 1— (a| 2 ), in which case it would be necessary 
to introduce the symbol 1 into the equation in an explicit form. But these difficulties 
disappear when we write the equation thus, 
+ [-(«| ! -l)(^-l)]=0. 
In this case we do not define the above event as a substantive event ; but we simply 
say that it is the reverse of another event, the nature of which we can readily compre- 
hend, the two alternative causes of which are the transference of (a£ 2 ) and the trans- 
ference of (0| 2 ). 
Again, the equation 
a + a X 2=2a X’ 
which expresses the formation of hydrochloric acid, may be written thus, 
«( Z -1) 2 =0, 
