In a paper recently communicated to the Society by Mr Main and myself, we showed that when nitrogen and aqueous vapour are passed over magnesium ammonia is formed. We took for granted the theory of the reaction to be that the magnesium decomposes the aqueous vapour, and that the hydrogen thus produced, when in the nascent state, combines with nitrogen and forms ammonia. In a discussion which followed the reading of the paper, Mr Dittmar maintained that such was not the theory of the reaction, but that nitride of magnesium is first produced, and then decomposed by the aqueous vapour into ammonia and magnesia.