Hello,

A mixture of calcium carbide and calcium phosphide placed in a container which reacts with water to produce gases like PH3 and C2H2, which burns and guides the ships in sea.  PH3 gas is called phosphine, and it is used in holmes signal. Holmes signal is basically the container containing calcium carbide and calcium phosphide gas.

1. When  calcium carbide and calcium phosphide mixture is kept in container it undergoes a reaction with the water liberating gases like PH3 and C2H2.
2. This gas helps guiding the ships that comes sailing in the sea.
3. It is also used in smoke screens.
4. Containers which have a perforated base and the hole at the top are filled with this mixture of calcium phosphide and clacium carbide and are thrown into the sea.
5. Water enters the container through the bottom and reacts to give acetylene and phosphine.
6. Phosphine gets ignited spontaneously as it comes in contact with air and also ignites aceteline. Thus a bright red flame is produced which is accompanied by huge smoke due to the burning of phosphine. This serves as a signal to the approaching ships.

Hope this helps!