I. _Volatile, in the form of Gas or Vapour._

1. Substances which, like the halogens, appear to form substitution

compounds, _e.g._

Chlorine,

Bromine,

Iodine.

2. Substances which probably combine chemically, and thus destroy

contagion:

Sulphurous acid,

Nitrous acid,

Fumes of other acids.