112 Condititions of Chemical Change and its Amount. [May 9, 



Variation of Temperature. 



The discussion of the numerous experiments made at temperatures 

 ranging from to 50, in media in which the quantities of iodide 

 range from 3'64 HI, to 23 HI, the quantities of hydrogen sulphate 

 from 45 H*SO* to 468 H 2 /S0 4 , and the quantities of hydrogen chloride 

 from 70 HCl to 547 HGl, leads to the following law of connexion 

 between chemical change and temperature. 



If 0.1 is the rate of chemical change at a temperature t in a homo- 

 geneous medium consisting of given constituents per unit volume, 

 and 2 is the rate at a temperature 2 in the same medium, the ratio 

 of i to xz is {(273 + ^)/(273 + ^)} m - ni being a constant depending 

 upon the character of the constituents of the medium. When the 

 temperatures are measured from the absolute zero 273, and are 

 denoted by Tj, T 2 , the formula assumes the simpler form, 



i/ 2 =(T,/T 2 ) w . 



The constancy of the value of m for a particular medium is secured 

 when the quantity of the predominant constituent of the medium is 

 sufficiently large in comparison with the quantities of the other con- 

 stituents to make the medium practically homogeneous. When this 

 is not the case m has some value intermediate to the values which it 

 has when one or other of the constituents is sufficiently predominant 

 to secure a constant value. 



In media in which hydrogen sulphate is sufficiently predominant, 

 the value of m is 20'38 ; similarly for hydrogen chloride the value of 

 m is 21'17. When the medium consists of water and hydrogen 

 iodide, the value of m is 24'1. The introduction of sodium sulphate 

 in large quantity into a medium otherwise consisting mainly of 

 hydrogen sulphate reduces the value of m from 2O38 to 18*1. In 

 a medium in which the main ingredient is sodium hydrogen carbo- 

 nate, the value of m is approximately 10. 



A further confirmation of the law of connexion between chemical 

 change and temperature is obtained from the discussion of experi- 

 ments on the rate of change of hydrogen chlorate and potassium 

 iodide made by W. H. Pendlebury and M. Seward. The value of m 

 is in the case of this chemical change 40'5. 



. It follows from the law enunciated above that at the temperature 

 of absolute zero no chemical change can take place. 



If the smallest value of in, viz., 10, is taken, a chemical change 

 which is completed in one minute at a temperature zero, would 

 require for its completion at a temperature of 200 a little more 

 than a year. If 20 is taken as the value of m, the minute would be 

 increased to more than half a million of years by the same reduction 

 of temperature. 



