1. During spectroscopic investigations of the phosphorus trioxide glow a method was found for maintaining a stationary glow for twelve to twenty-four hours.

2. Experiments were performed illustrating the necessity of water-vapour for the glow phenomena.

3. The effect of water-vapour on phosphorus trioxide in evacuated bulbs was systematically studied at 25°. Pressures of water-vapour ranging from 1–23 mm. were maintained by the use of certain salt hydrate mixtures, and of water.

4. In every case pressure increased owing to the production of phosphine.

5. When the pressure of water-vapour was 2·5 or 5 mm. a liquid, in addition to gas, was produced in sufficient quantity, after the elapse of several hours, to cause a rapid pressure rise to a maximum. Its vapour-pressure at different temperatures showed that it was liquid phosphorus hydride. A rapid pressure fall from the maximum was observed, and was due to the interaction of hydrides of phosphorus and phosphorus trioxide.

This investigation was proposed by Sir James Walker, F.R.S., to whom the author is deeply indebted for many valuable suggestions, as well as for his constant interest and kindly criticism. She has also to express her thanks to Messrs Brunner, Mond & Co., Ltd., for a grant which defrayed the cost of a Cenco Hyvac Pump and other apparatus.