Holme signal uses chemical compound

In case of emergency help to a ship. a box containing CaC₂ and Ca₃P₂ is thrown in water using a canon. The reaction with water produces Ca₂H₂,  PH₃ and P₂H₄. The compound P₂H₄ is self  combustible and burns PH₃ and C₂H₂, White smoke of P₂O₅ and black smoke (incomplete combustion of C₂H₂) is visible at long distance indicating demand for help.

${\mathrm{CaC}}_2+2{\mathrm{H}}_2\mathrm{O}\stackrel{ }{\to }\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+{\mathrm{C}}_2{\mathrm{H}}_2$

${\mathrm{Ca}}_3{\mathrm{P}}_2+6{\mathrm{H}}_2\mathrm{O}\stackrel{ }{\to }3\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+2{\mathrm{PH}}_3$