of Edinburgh, Session 1868-69. 5o7 
described is a point of considerable interest, I have thought it right 
to embody the remarks which I then made in the note now pre- 
sented to the Society, in order to remove the doubt which I find 
exists in the minds of some as to the theory of the reaction. 
That the ammonia is not due to the action of aqueous vapour 
on nitride of magnesium is, I think, clearly shown by the follow- 
ing considerations : — 
(1.) Nitride of magnesium is a very unstable compound, and is 
stated to decompose water with such violence that it makes the 
water boil. A priori it is a very strong objection to an explana- 
tion which assumes the formation of a body under the very condi- 
tions which decompose it. To say that nitride of magnesium could 
be formed in the presence of aqueous vapour, is pretty much the 
same thing as to say that potassium could be formed in the pre- 
sence of water. It is certainly the duty of those who make a sup- 
position so entirely at variance, as I think, with dynamical con- 
siderations, to support it by some experimental evidence. 
(2.) Nitride of magnesium is only formed when magnesium is 
heated to a high temperature in presence of nitrogen. But 
ammonia is formed in small quantity in our experiments when 
nitrogen and aqueous vapour are passed at low temperatures over 
magnesium. In this case, it is certain that nitride of magnesium 
is not produced ; for it would not be produced if aqueous vapour 
were not present, and it surely will not be pretended that the pre- 
sence of aqueous vapour might induce the formation of nitride of 
magnesium under conditions in which it would not be formed in 
its absence. 
It is equally certain that at low temperatures magnesium de- 
composes aqueous vapour with formation of hydrogen ; and in the 
union of nascent hydrogen and nitrogen, we have a sufficient, and, 
as I maintain, the only explanation of the formation of ammonia. 
(3.) To suppose that nitride of magnesium could be formed is 
equivalent to maintaining that magnesium has, in the conditions 
of our experiments, a greater affinity for nitrogen than for oxygen, 
which the instability of the nitride and the great stability of the 
oxide of magnesium shows is not the case. This reason alone is 
sufficient to negative the supposition of the formation of nitride of 
magnesium. 
