432 



Dr. Waller's Experiments on the 



sure by varying the interval of the iodine from the metal, or 

 by interposing at some distance from its surface a disc of pa- 

 per so as to cause the vapours of iodine to pass through it. 

 Bromine may be made use of likewise by pouring a few drops 

 of it over some carded cotton, and using it in a similar man- 

 ner with the iodine. In respect to chlorine, it is most con- 

 venient to disengage it slowly by dropping a little sulphuric 

 acid upon some chlorinated lime. 



In illustration of the objects of this mode of experimenting, 

 I will adduce some of the results it has given me with various 

 metals. Some of the experiments below were performed be- 

 fore I had the idea of watching the progress of the combina- 

 tion through a transparent medium ; they are therefore less 

 exact than they might otherwise have been : but I have pre- 

 ferred stating them as I had inserted them in my note-book 

 befoie I had conceived any idea as to their probable utility in 

 the elucidation of chemical affinity, and when I intended them 

 for other purposes, which I shall hereafter explain. 



