Viridigen is the name given to the green gas associated with Martian technology. According to the Martian Technology Report:
Throughout the Martians’ time on Earth, one consistent element of their engines and machines has been the presence of green smoke or flame, and flashes of green light. A distinctive property of the cylinders as they fell to Earth was the trail of green vapour they left as they passed through our atmosphere, and as has been mentioned previously, flashes of green light and clouds of green smoke have been associated with their industrial activities. This green gas has been labelled viridigen. As noted before, it has the peculiar property that, when subjected to an electric current, it undergoes expansion. It seems that the individual particles of the vapour become highly charged, repelling each other with great force, and fluorescing at the same time. A subsequent property of this relationship between the pressure of the gas and electrical charge is that when the pressure of the gas is changed, it generates an electrical charge.
This pressure change can be accomplished mechanically, for example by compressing the gas in a piston, or thermally, by heating or cooling the gas whilst holding the volume constant. The nature of the charge depends upon the nature of the pressure change. An increase in pressure generates a negative charge, whilst a decrease in pressure generates a positive charge. It appears that the Martians generally used the former method of electrical generation, heating the gas in a container of fixed volume, which, according to Boyle’s laws, increases the pressure inside that container and causes the viridigen to generate a negative charge.
This indicates that Viridigen is a peculiar green gas that expands when exposed to electricity, and can also generate an electric charge when it's pressure is changed.