156 Chemistry. [Lecture 29. 



gradually disappears. Some have attempted to 

 explain this phenomenon by saying that the 

 colour is owing to a small quantity of copper 

 combined in the nitric acid, for saline liquors 

 are found to produce a green colour with cop- 

 per ; but this is not the case. This colour on 

 mixing the acid with water is not an inseparable 

 quality. 



It was mentioned (Lect. XXVII.) that there 

 are two states in which this acid appears: 1st, 

 The nitric, or the base fully saturated with oxy- 

 gen; 2dly, The nitrous, or the nitrogen com- 

 bined with a smaller proportion of oxygen. 

 Such is the aquafortis of the shops, which is not 

 only weak but impure. It is, however, a sub- 

 stance of considerable importance in the arts, 

 particularly in dyeing, etching on copper, and 

 assaying ores and metals. Nitric acid, in its 

 strongest state, is even more corrosive than the 

 sulphuric. 



This acid with ice or snow is used to produce 

 one of the freezing mixtures. There is no dif- 

 ference between snow and ice in the effects, ex- 

 cept that the snow being more divided, gives a 

 greater extent of surface for the acid to act on. 

 Such an intense degree of cold is produced by 

 the mixture, that if the glass is breathed upon, 

 the breath is instantly frozen, and forms a hoar 

 frost. The same colour takes place on the snow 

 being dissolved, as when the acid is mixed with 

 water. 



