THERMOGRAPHY AND VOLTAISM. 301 



THERMOGRAPHY AND VOLTAISM. 



Lecture at the London Institution. 

 Literary Gazette, Jan. 21, 1843. 



THE following facts Mr. Grove had himself observed with 

 regard to metallic thermographs, i. When two portions of the 

 same metals are juxtaposed each being at the same temperature 

 little or no effect is produced. 2. When at a different tempe- 

 rature a slight effect is perceptible. 3. When the metals are 

 different at the same apparent temperature a greater effect is 

 perceptible. 4. When the metals are different, and at different 

 temperatures, a still greater effect is visible. Again, taking, 

 for example, a sovereign placed on a silver or copper plate, the 

 effect is increased if the coin be breathed on before being 

 brought into contact ; it is still more increased if the coin be 

 rubbed with oil and wiped apparently dry ; and it is still in- 

 creased if it be held for an instant over the vapour of a 

 substance capable of chemically acting on the juxtaposed plate ; 

 as, for instance, over ammonia, before being placed on copper. 

 These facts led him to believe that such impressions were 

 caused by a radiation and condensation of whatever vapour 

 exists between the metals, and which, by being condensed 

 unequally (in the case of different metals, by their different 

 conducting and radiating powers), produce an impression 

 according to the unequal distance of the different parts of the 

 stamp on the coin. In fact, if we rightly understood him, the 

 phenomenon was in some degree analogous to dew. Be this 

 as it may, whatever the nature of the radiation be, there is 

 unquestionably some radiation ; and this fact of inter-radiation 

 was applied by Mr. Grove to explain a fundamental experi- 

 ment in voltaic electricity, hitherto a subject of much contro- 

 versy, being the effects of Volta's original experiment on the 

 contact and severance of two discs of dissimilar metals. The 

 contact theory of electricity attributes the effects on the elec- 

 troscope produced by this instrument to simple contact ; the 



