Acetylene is an extremely flammable gas and can form an explosive atmosphere in
the presence of air or oxygen. Acetylene poses an additional hazard to other
flammable gases as it is also unstable. Under certain conditions, even in the
absence of any air or oxygen, it can decompose explosively into its constituent
elements, carbon and hydrogen.
An acetylene cylinder has a different design from most other gas cylinders. It
consists of a steel shell containing a porous mass. The acetylene gas in the
cylinder is dissolved in acetone which is absorbed by the porous mass.
Decomposition of the acetylene is triggered by heat, eg when it is:
AplusA-online.de - Source: Health and Safety Executive