ON AQUA REGUA AND HYPOSULPHURIC ACID. 207 
ON THE NATURE OF AQUA REGIA. 
16. Aqua regia is very peculiar in its characters and 
functions. Its formation is especially influenced by the 
temperature, by the degree of concentration of the two acids 
from which it is formed, and by the presence of a body ca- 
pable of combining with chlorine. 
17. By mixing hydrochloric with nitric acid, both being 
concentrated, and the temperature very low, no reaction is 
caused. The cold liquids are mixed without colouration or 
disengagement of chlorine. If, however, the mixture is 
removed from the refrigerating medium, and put in a place 
in which the temperature is 32° F., it gradually becomes 
coloured, and chlorine is disengaged. 
18. A mixture of two volumes of hydrochloric acid of 
37.60 per cent., and one volume red nitric acid, forms chlo- 
rine only at a temperature above 9° F. Nitric acid, con- 
taining 74.5 per cent, of real acid, being treated with the 
same hydrochloric acid, is coloured, and forms chlorine, at a 
temperature above 21° F. 
The presence of a greater amount of water may still 
further retard the reaction. Finally, if the acids be very 
much diluted, even by the application of heat, no reaction 
is caused ; if, however, in the latter case, a metal be thrown 
into the liquid, a reaction follows, and water is formed. 
Soon afterwards, this liquid diminishes the affinities to such 
an extent, that the reaction would entirely cease, if a metal 
of the sixth class — for instance, gold — were made use of. 
19. These phenomena prove that the formation of aqua 
regia is by no means the effect of the mutual reaction be- 
tween the elements of hydrochloric and nitric acids (the lat- 
ter being considered as composed of N 2 4 -f 0,) but that it is 
to be attributed to the action of the hydrogen of the one 
upon the oxygen of the other. To produce the effect, it is 
ndeed requisite that, either by the application of heat, or 
the use of a body having great affinity for chlorine, or these 
