1122 EFFECT OF GALVANISM IN METALLIC ARBORIZATIONif. 



btate : the hidrogen being on the contrary repelled, because 



it also acquires the vitreous electrity, attracts^ for the same 



reasoQ the oxigen of the metallic oxide held in jsolution by 



the acid uud water ; whence it foUov^s, that the metal, being 



divested of its oxigen, imibt reappear in its perfect lustre. 



Qtiaiuity of ^ circumstance not to be forgotten in this explanation, but 



caloric in ati to which I cannot at present assign its just value, is the vari- 

 oxide piobd- , , . , , . • I I 11' -1 T 



biy proporti- ^ble quanti|ty at caloric retained by metallic oxides. It 



onate t«> the appears probable to me, that this quantity is in the inverse 

 the metal. ratio of the oxidability of the metal. The following is the 

 manner in which Berthollet expresses himself on this head 

 in his excellent Essay on Chemical Statics, vol. II, p. 382. 

 BerthoTlet's ** Oxigen retains a larger or smaller quantity of caloric in 



lemark jts combination with metals, as well as in those it forms with 



other substances. On this depend some of the properties, 

 that distinguish oxides in their relation to combustible sub- 

 stances. Those of gold, silver, and mercury, retain a great 

 deal; whence their compounds with ammonia detonate 

 either by a slight rise of temperature, or even by com- 

 pression. The oxide of copper, which also is rendered 

 capable of decomposing ammonia by an increase of tem- 

 perature, produces however no drtonation; which shows, 

 that the oxigen in it is much more def)rived of caloric. 

 The oxide of silver detonates more forcibly than that of 

 gold, or of mercury ; so that the oxigen appears to retain 

 more caloric in the former, than in the latter.*' 

 questionecT by Brugnatelli, whose sagacity is generally acknowledged, 

 BrognatelU. objects to this theory, that we observe no evolution of caloric 

 ivhen silver is revived by means of copper; and thence he 

 concludes in favour of his own theory of' therraoxigen. 

 This observation appeared to me so decisive, that I had com- 

 pletely adopted the opinion of Brugnatelli in this respect. 

 Evolution of However, reflecting one day on this interesting subject, I re- 

 ways percep- collected, that a thermometer, placed under a receirer in 

 'iWe : which a vacuum is made, is but little affected by the cold 



produced^by the dilatation of the air, and it occurred to me, 

 that the calonc, which is extricated in the precipitation of 

 one metal by another, may be so quickly absorbed by the 

 9\irTound»ng bodies, as not to become sensible. Hence I con- 

 ceiredj th^t its dispersion might be preveuted by dihiibish- 

 U inff 



