coloured Films formed by Iodine, Sfc. upon Metals, 431 



power in the various phaenomena of solution and precipitation 

 of saline substances, and according to him what is called inso- 

 lubility in a body is merely the result of its strength of cohe- 

 sion, an entirely physical property. 



When the intervention of caloric is required, the effects are 

 still more complicated, as they vary according to the intensity 

 of the heat employed, and the time its action is exerted ; be- 

 sides, the chemical action when it does take place is frequently 

 so instantaneous that it is impossible in our present state of 

 science to imagine any means by which it might be measured. 

 In the combination of the three bodies, iodine, bromine and 

 chlorine, with the metals, however, most of these objections cease 

 to exist, or may be easily avoided. As their vapours com- 

 bine with the metallic surfaces at the ordinary temperature, 

 they are all of them in the same circumstances in that respect ; 

 and if the temperature should be required more elevated, the 

 gasiform state of these substances, iodine not excepted, en- 

 ables us to submit the metals to be experimented upon all 

 at the same time to the same influence. If, therefore, it were 

 possible to reduce the metallic substances into fine powders 

 the particles of which were of the same dimensions, by acting 

 upon them with either of these vapours, an idea might be 

 formed of the affinities which produce their binary com- 

 pounds by the increased weight acquired by the powders in 

 this process ; but the difference which exists in the physical 

 properties of the various metals would preclude the possibility 

 of any near approach to accuracy in this mode of proceeding ; 

 but by acting on the polished metallic surfaces, as in the pre- 

 ceding experiments, all the advantages offered by the process 

 with the powders are included, whilst several of the difficulties 

 are removed. As the film of the compound augments, it un- 

 dergoes the various changes of colour which take place in all 

 transparent films, thus affording a means of ascertaining the 

 absolute thickness obtained in different circumstances, when 

 it would be difficult to detect the slightest difference in weight 

 by means of the most delicate balance. The depth of this 

 coating may be ascertained when either the index of refrac- 

 tion of the compound itself is known, or if the angle of po- 

 larized light is given by means of the law discovered by Sir 

 David Brewster, between the tangent of the angle of polari- 

 zation, and the index of refraction. The most convenient 

 way which occurred to me of performing these experiments, 

 was the employment of a bell-glass within which some iodine 

 is fixed at the top ; this apparatus being placed over the metal 

 to be acted on, the experiment may be watched in all its pro- 

 gress, and the action can be retarded or accelerated at plea- 



