254 Dr Hollis, Note on the Pulverization of 



acids. After a while, however, the surface of the sheet loses its 

 brilliancy, becomes etched in fact, and occasional bubbles form, 

 especially upon the under surface of the metal, which show that 

 some slight reaction takes place between the acid and the metal ; 

 particularly is this the case when the strong acid is used. As far 

 as my experience carries me nickel wire shows the passive con- 

 dition more markedly than even the sheet nickel does, whenever 

 the former is immersed in the acid. 



It is, however, with the behaviour of nickel-grains under 

 similar conditions that this note is mainly concerned. If a few 

 drops of fuming nitric acid are placed in a test-tube and a 

 nickel-grain is dropped into it, one of three results follows. 

 (1) Immediately, or after a short interval, violent ebullition 

 occurs, with a considerable rise in the temperature of the mixture, 

 and the copious disengagement of reddish brown vapours. In 

 this case the ebullition continues and the metal is irregularly 

 eaten away, drilled it may be into holes by the action of the acid, 

 until a solid mass of green crystalline nitrate has replaced the 

 liquid surrounding the metal core. (2) The nickel-grain upon 

 immersion in the acid assumes the passive condition, and no 

 chemical reaction is observable. (3) A critical state of unstable 

 equilibrium is set up apparently in the metal itself. As a result 

 a slow disintegration of the metal ensues and a powdery metallic 

 precipitate falls to the bottom of the tube. At the same time 

 some chemical reaction takes place, — in contrast to the physical 

 changes above mentioned, — gaseous bubbles are more or less 

 rapidly formed and the liquid assumes the green tint of the dis- 

 solved nitrate. If the chemical reaction becomes too vigorous, 

 a sudden change to the ebullition stage takes place and (1) is 

 initiated. In rare cases after a while (3) is succeeded by the 

 passive state (2), and the experiment ends unsatisfactorily. As 

 I was anxious to prolong the unstable stage so as to obtain more 

 decided results than had been possible heretofore, some ex- 

 periments were made with that object in view. It was eventu- 

 ally found that when a thin layer of fuming nitric acid was 

 spread over a glass dish and a few nickel-grains so placed that 

 their lowest surfaces were alone in contact with the acid, the 

 greater number of them assumed state (3), and a considerable 

 quantity of the powder was obtainable. Any objectionable grains, 

 which refused to conform, if I may so put it, to the rules of the 

 laboratory, were at once removed and the others allowed to remain 

 so long as any reaction took place. 



The powder so obtained is in colour a grayish white, with 

 a dull metallic lustre. It is not pyrophorous, but is strongly 

 magnetic. Its structure is presumably crystalline. At all events 

 many of the grains when magnified are seen to be somewhat 



