CHLORINE, BBOillNE, IODINE, AND FLUOBINE. 167 



Fig. 70. 



You see the chlorine of the sodium chloride unites with 

 part of the hydrogen of the sulphuric acid, forming hy- 

 drochloric acid ; and the sodium takes the place of the hy- 

 drogen which left the acid, forming an acid salt hydro- 

 sodium sulphate. Now you see what would have taken 

 place had we omitted the manganese dioxide in the prepara- 

 tion of chlorine in 211. Compare the two equations. 



225. Aqua Regia. This is a mixture of nitric and hydro- 

 chloric acids in the proportion of one part of the former to 

 three of the latter. Neither of these acids alone will dis- 

 solve gold or platinum, but this mixture of them will do it ; 

 and as gold is considered the king of metals, the liquid that 

 can dissolve it has been styled aqua regia, or royal water. 

 But we do not have in this case, in reality, a mere physical 

 solution of gold. It is something more. A chemical change 

 takes place by which a union is effected between the gold 

 and the chlorine of the hydrochloric acid, making chloride of 

 gold, and it is this salt of gold which is dissolved, and not gold 

 itself. The explanation is this : While gold put into hydro- 

 chloric acid can not take its chlorine, the nitric acid which 



