402 C. E. FAWSITT AND A. A. PAIN, 
centage of H.SO. in the acid is 96°08. A concentration 
of 84°5% acid corresponds to the composition of the mono- 
hydrate He2SO., H.O and so will not gain or lose in con- 
centration when acting. More dilute acids will lose in 
concentration. 
As the acids employed usually had a much smaller action 
in these experiments than that formulated in the calcula- 
tion, any alteration in the concentration of the acid from 
the initial values may be regarded as negligible. 
In a previous research’ it was suggested that the real 
(maximum) velocity of the action of concentrated sulphuric 
acid might not always be obtained owing to the adhering 
coats of ferrous sulphate and other causes. In many of 
those cases, shaking appears to be all that is necessary to 
produce the maximum velocity. 
In many of the experiments, the results of which are given 
below, the velocity of action increased continually towards 
the maximum as time advanced, even when the flasks were 
not shaken; one could explain this by assuming that the 
faster the evolution of hydrogen gas, the more stirring 
action there is of the acid at the surface of the iron exposed; 
or one could assume that the products of the action have 
some accelerating effect on the action. It was assumed in 
a previous research, using the same steel and 94°67 acid, 
that the velocity of action obtained at 30° C. viz. 2°4 cc. 
per sq. dcm. per hour, approximated pretty closely to the 
the maximum velocity obtainable. In the first experiment 
(Series A, Table II) the flasks were left quite undisturbed 
except for the stirring action of the gas evolved, and the 
velocities obtained with 94% acid were somewhat less than 
those obtained originally by Fawsitt and Powell (loc. cit.) 
with 94°6/% acid. The concentration of acid in those solu- 
tions was determined in the first instance by density, but 
1 Journ. Chem. Soc. Ind., xxx111, 234, 1914. 
