1895.] Conditions of Chemical Cliange and its Amount. 109 



rate of change per unit of sulphate per unit of iodide is constant 

 when an unit volume contains quantities of sulphate ranging from 45 

 to 515J? 2 jS0 4 .* This increment is again constant for the range 515 

 to 762H-S0 1 , but has a higher value. For quantities of sulphate 

 greater than 762 H*S0 4 this increment, which again becomes con- 

 stant, is further increased in value. The explanation given of this 

 discontinuity in the successive values of the increment, is that the 

 gradual addition of the sulphate to the water gives rise to the gradual 

 formation of one hydrate at the expense of another, and the increment 

 per unit of sulphate is the resultant of the increment per unit of each 

 of the hydrates. 



For the range 515 to 762 H*SO* the hydrate which is decreasing, 

 is H 2 S0 4 , 106 H 2 0, and the hydrate which is increasing, is H 2 S0 4 , 

 71 H 2 O. 



At a temperature of 30 the values of in a solution containing in 

 1 c.c. 1 HI, sH^SO* are 



( 1 ) from s = 45 to s = 515, 

 = (1230 + 18'1 s) 10~ f> ; 



(2) from s = 515 to s = 762, 



= {1230-1- 18-1 X 515 + 22-4(> 515) }lQ- 6 ; 



(3) from s = 762 to s = 1140, 

 * = {1230 + lS'l X 515 + 22-4(762 515) +26'5(s- 762)}lO~ 6 . 



Theoretically the rate 1230 X 10~ 8 is the rate with water and 

 hydrogen iodide only, present in amounts per cubic centimetre 

 55400 H^O, \HI. The increments per unit of hydrogen sulphate 

 are in the three cases 18'1, 22'4, and 26'5 millionths. 



When the medium consists of water, hydrogen iodide, and hydro- 

 gen sulphate, and the ratio of the numbers of K~SO l and HI falls 

 below 20, the rate with unit of HI has a value different from 1230 x 

 10-'; but the value of the increment per unit of sulphate is the same 

 for the range of numbers of H^SO 1 from 45 to 515. 



Variation of Hydrogen Chloride. 



It is shown from the results of one set of experiments at a tem- 

 perature of 30 that the increment in the value of the rate of change 

 per unit of chloride per unit of iodide is constant when a cubic 

 centimetre contains 14 HI, and quantities of chloride ranging from 70 

 to 280 HCl, the value of the increment being 16'8 X 10~ 6 . An examina- 

 tion of four sets of observations made in the course of the experiments 



* As in our former papers, we use H to represent one-millionth of a gram or 

 hydrogen, and other symbols italicized for the corresponding proportions of other 

 elements. Thus H*SO 4 is 98 millionths of a gram of hydrogen sulphate. 



