of Chemical Change. 123 



it is clear that, other conditions being the same, the constant 

 b is inversely proportional to the amount of free acid, and a 

 represents the number of units of time required to oxidize 

 half the original iron. Experiments show a to have this value, 

 as may be proved thus : — Let Y be the value of y when £=0, 



then b= Y a. When y is reduced to one half, Y a = -^-(a + 1) ; 



therefore a = t. 



Experiments. — The iron solutions employed were prepared 

 from ferrous sulphate which had been recrystallized twice, 

 and contained about 2 per cent, ferrous iron and 10 per cent, 

 hydric sulphate, together with a small quantity of ferric iron. 

 When not in use they were kept in an atmosphere of coal-gas. 

 The potassic chlorate was repeatedly recrystallized until free 

 from chlorides and sulphates. The experiments were gene- 

 rally made in sets of four, two blanks containing always the 

 same amount of acid, and two having their acid some multiple 

 of that in the blanks. The mean values of the constants a, b, 

 were then compared. In this way any slight fluctuations in 

 the temperature of the water-bath were eliminated. The 

 method of making the experiments was as follows : — After all 

 the solutions had acquired as nearly as possible the temperature 

 of the room, the required amount of hydric sulphate was run 

 into a 250-cubic-centim. flask, the iron solution was next 

 added, and the whole made up to the mark with distilled water 

 that had previously been boiled to expel air. The mixture 

 was then decanted into an ordinary flask of about 600 cubic 

 centims. capacity and placed in the bath. When the four 

 solutions had been made up in this manner, the potassic chlo- 

 rate was run in with constant agitation and the time noted to 

 two tenths of a minute ; five minutes were added to this reading, 

 and taken as £ = 0. The two observations from each solution 

 required for the formula were taken, the first about 8 minutes 

 after the addition of the potassic chlorate (or £ = 3), and the 

 second when about 20 per cent, of the iron had been oxidized. 

 The thermometer used was divided into tenths, and could easily 

 be read to *05° ; all the burettes, pipettes, &c, were calibrated 

 and expressed in terms of each other. 



The first series of experiments (contained in Table I.) were 

 performed with the following solutions — 



( ferrous iron 1*735 per cent. 

 Ferrous sulphate-? ferric „ '030 „ 



(free H 2 S0 4 9*86 „ . 

 Potassic chlorate 1*813 per cent. KC1 3 . 



Each experimental solution consisted of 28*7 cubic centims. 



