Kohlenberg—Action of Metallic Magnesium, Etc. 309 
other solutions and so the attempt he made to generalize this 
explanation. In addition to the ammonium] salts, there are 
several eases that might he considered to favor this view. So 
magnesium hydroxide is less soluble in KOH and HaOH so¬ 
lutions than in pure water, and the fact that these solutions do 
not attack the mjetal as much as does water might he regarded 
as a support for the view advanced. Again, since ammonium 
fluoride solution is not acted upon by magnesium, and since the 
hydroxide and the fluoride of magnesium are insoluble in so¬ 
lution of ammonium fluoride, this might he. urged as another 
striking instance of the same kind. It would, however, he a 
delusion to think that the insolubility of magnesium hydroxide 
in these solutions is what prevents the magnesium from acting 
upon them'. This becomes evident from the following experi¬ 
ment: On making magnesium amalgam (by heating magne¬ 
sium and mercury together) and treating normal solutions of 
KOH and HaOH with the same, I found that hydrogen, is very 
rapidly evolved, hydroxide of magnesium) is formed and mer¬ 
cury set free from] the amalgam. Solution of fluoride of am¬ 
monium is also violently attacked by magnesium amalgam with 
concomitant liberation of hydrogen. Mere contact of magne¬ 
sium and mercury in these solutions will not bring about this 
action; the amalgam must be used. 
At a given temperature and pressure the course that a chem 
ical reaction will take is determined (1) by the chemical affin¬ 
ity between the reacting substances and (2) by the relative 
masses of these substances. If the product is an insoluble one 
and closely envelops the surface of the solid that is being acted 
upon, the rate of the action will be diminished because of dimi¬ 
nution of the surface exposed. This effect of the accumula¬ 
tion of the insoluble product of the. reaction increases, the 
longer the reaction goes on, and may finally practically check 
the process ; which is clearly shown by the results of Lemoine 
cited above. While there can be no doubt of this effect, never¬ 
theless, the differences in the rates of evolution of hydrogen 
during the times recorded above are in general too great to be 
accountd for solely on the basis of accumulation of the precipi¬ 
tate. Of this I became especially convinced by treating aque- 
