766 MR. BRODIE ON THE CONDITION OF CERTAIN ELEMENTS 
4. We know from the researches of H. Rose, that in a neutral solution of the bi- 
chloride of mercury, the mercury is precipitated on zinc in the form of a black pow- 
der, but on the addition of an acid the metal becomes bright and the zinc is amalga- 
mated*. The addition of bichloride of mercury also instantly stops the most violent 
evolution of hydrogen from the action of hydrochloric acid upon zinc, with the for- 
mation of the same amalgam. These curious experiments, which certainly caused 
much surprise in Rose, become intelligible from the considerations which I have 
mentioned. The combination of zinc with mercury is a combination in a different 
direction to that of the zinc with chlorine, hence the latter determines and causes the 
first ; thus — 
+ — + — -l- — 
H Cl Zn Zn Hg C1 = H Cl+Zn Hg-f Zn Cl. 
V ; 
5. A very beautiful illustration of the way in which one chemical combination de- 
termines another, is seen in another experiment with the same metal. Zinc, it is 
known, decomposes slowly, with evolution of hydrogen, a strong and boiling solution 
of caustic potash ; but when into the solution a few crystals of nitre or nitrite of 
potash are thrown, the zinc is rapidly dissolved with the evolution of ammonia. The 
reason of this I consider to be, that the polar division of the zinc now takes place 
with the greatest facility, the zinc being oxidized in two directions ; the zinc decom- 
poses the water, by which a polarity is given to the hydrogen, w^hich causes it to 
combine with the nitrogen of the nitric acid to form ammonia, by which combination 
the other particles of the nitric acid become- in their turn polar and oxidize again the 
zinc ; the action, as it were, proceeding from and returning back again to that ele- 
ment. The change, for example, may be represented thus: — 
+ — ~ + — 
Hg Og ZUg ZOg Og N = NHg -{- fiZuO. 
V ) 
6. To the preceding experiments I wdll only add the phenomena of substitution, 
which, viewed in relation to them, assume a new form. The division of the chlorine 
w^hich takes place in substitution is a very similar fact to that distribution of the ele- 
ment which takes place in the formation of chloro-sulphuric acid (which I have be- 
fore cited), and admits of a similar explanation. The problem is, why, with even 
the smallest quantity of chlorine, does a portion of the chlorine ever combine with 
the organic body? Why is the hydrogen never simply removed ? The solutionis, 
that one of these two changes determines the other, precisely as the combination with 
olefiant gas determines the combination with sulphurous acid. Nor do I see wdiat in- 
telligible distinction is to be drawn between the action of chlorine on the organic 
body and that of hydrogen on nitric oxide, or between either of these two cases and 
that of water itself on chloro-sulphuric acid. The organic body behaves as though 
it were a simple compound of hydrogen on the one hand, and the remaining ele- 
* Liebig’s Annalen, vol. Ixiv. 28.3. 
