ix SULPHUR AND PHOSPHORUS 167 



The marked difference between vitriolic acid air and oil 

 of vitriol was attributed by Priestley to the fact that the oil of 

 vitriol had "combined with phlogiston." Lavoisier, how- 

 ever, showed that the change was due to removal of oxygen 

 from the oil of vitriol. 



Priestley converts " vitriolic acid air " into oil of 

 vitriol. Having prepared solutions of " vitriolic acid air," 

 Priestley showed that they resembled oil of vitriol in their 

 power of dissolving metallic zinc (ibid. p. 274). He also 

 found that the volatile or " phlogisticated " acid could be 

 reconverted into ordinary oil of vitriol by exposure to air, 

 the acid being held in solution during the experiment by 

 combination with alumina. 



" The volatile vitriolic acid, though produced from the fixed 

 vitriolic acid, is very considerably different from it, especially 

 as it may be dislodged from its basis by the vitriolic acid, 

 just as other weaker acids are dislodged by those that are 

 thence called the stronger. But that volatile vitriolic acid is 

 capable, however, of being brought back to the state of the 

 common vitriolic acid, and becoming the same thing that it 

 originally was, several experiments shew. At the time of 

 my last publication I had found that it was capable of dis- 

 solving iron and zinc, and of producing inflammable air, 

 which is the property of oil of vitriol : but I had a more 

 decisive proof of the same thing when, to water saturated 

 with vitriolic acid air, I had, for another purpose, put 

 some earth of alum till it was saturated. For, after six 

 months, in which this solution had been exposed 

 in an open phial, and one third of it was evaporated, 

 I observed many transparent crystals formed at the 

 bottom of the phial, as well as an incrustation on the sides 

 of the phial above the surface of the liquor. These crystals 

 were all triangular, of a considerable thickness, connected 

 with each other, and when examined appeared to be alum, 

 which is known to be the saline substance formed by the 

 same earth, and the proper vitriolic acid " {Experiments 

 and Observations, 1779, IV. 122-123). 



