338 APPENDIX 



The following series of lecture experiments, taken from the notes of one of Pro- 

 fessor Loeb's lectures on physical chemistry, may find a fitting place here, at the 

 close of the elementary experiments. The outcome illustrates the point of view taken 

 by the first paper (pages 1 to 20) of this volume. [EDITOR.! 



I DESIRE to show you a series of experiments devised by Pro- 

 fessor Landolt, of Berlin, illustrating a class of phenomena that 

 are grouped together under the name of Speeds of Reaction. In 

 this particular case we shall examine the rapidity with which a re- 

 action takes place between iodic acid and sulphurous acid. In the 

 first place, the sulphurous acid serves to reduce the iodic acid to 

 hydriodic acid, which as soon as the sulphurous acid is exhausted 

 will react with any remnant of the iodic acid, producing water and 

 free iodine. Up to the moment that the last particle of sulphurous 

 acid has disappeared, or rather, been converted into sulphuric acid, 

 it is impossible that iodine should be separated in the free state; 

 consequently the appearance of this free iodine will be an indication 

 of the time it takes for the completion of the reaction between the 

 original substances. If this reaction were instantaneous, the ap- 

 pearance of iodine would immediately follow the mixing of the two 

 solutions; but you will soon see that this is not the case. Time is 

 necessary for the molecular exchange, and in this case, the interval 

 is sufficiently great to be readily measured by means of an ordinary 

 watch. 



In order to enable you to detect the iodine the moment it is set 

 free, I shall mix with the solution a little starch paste, which, as 

 you well know, gives a blue color with free iodine. Perhaps, how- 

 ever, I had better show you this in the first place. I take here some 

 starch paste in three beakers; to the first I add some iodic acid, 

 to the second some hydriodic acid and to the third a solution con- 

 taining a little free iodine. You see that only the third beaker shows 

 a decided blue color; iodine in combination not affecting the starch 

 in this way. 



We are now ready to begin with the experiment on speed 

 of reaction. In the first place, I measure out 25 cc. of iodic 

 acid solution in one beaker and 25 cc. of sulphuric acid into 

 another. The iodic acid solution being slightly stronger, we shall 

 have a little excess of this reagent, to which I add 5 cc. of starch 

 paste solution and 20 cc. of water. I shall now mix the two solu- 



