Re: how was plasma discovered

The answer I am to give you is maybe related more with the 'When' rather 
than with the 'How', but I hope you'll find it interesting anyway.

It could be surprising to you but, as the other three common states of 
matter (solid, liquid and gaseous), the Plasma State was 'discovered' by 
the first human beings on Earth. This is true if you consider that 'to 
discover' is just to realize there's something new and unseen, even if 
you can't explain or understand it. Plasma State appears on Earth in 
three basic forms: lightning, fire and Aurora Borealis (Northern Lights). 
Them all are plasmas as they are made of partially ionized gases (that's 
what a plasma is), and were known by prehistoric humanity.

Greek philosophers (400-500 B.C.) gave the first 'theory' of matter, 
establishing  that everything was made of a mixture of four elements: 
Earth, Water, Air and Fire. 
Because of the properties they gave to those elements we could compare 
them with modern states of matter:

Earth	-> Solid State.
Water	-> Liquid State.
Air		-> Gaseous State.
Fire	->  Plasma State.

As in many other subjects, greek philosphers made a great job on 
identifying and clasifying matter states and properties with nothing but 
their intuition and intelligence (they used no technology or 
experiments).

But if we have to be strictly scientifical in the modern sense, the 
privilege of discovering the Plasma State has to be given to the English 
scientist W. Crookes, who, in the late-middle 19th. Century, was studying 
the effect and behaviour of gases at low densities when an electric 
discharge were forced inside a tube filled with them. He realized that a 
new medium appeared such that an electric current could pass through it 
(gas inside the tube became ionized and therefore electrically 
conducting, i.e. plasma were created). By then he wrote 

''...The phenomenon in these exhausted tubes reveal to physical science a 
new world, a world where matter may exist in a fourth state ...''.

So he was the first on describing an ionized gas as a new state of 
matter. The answer to your question would then be that the Plasma State 
was discovered in the 19th Century while studying electric discharges in 
low density gases.

But the answer wouldn't be complete without mention of many other 
scientist who, some years later, developed Plasma Physics.

First of all I should mention Irving Langmuir. He was the first on giving 
a complete and appropiate theory of ionized gases, and was also the first 
on using the name 'Plasma' to refer to this new medium or state of 
matter. This happened by 1920's, when some other scientists (L. Tonks, R. 
Seeliger, B. Klarfeld, M. Steenbeck ...) were also studying inozed gases. 
By that time other physicists (Saha, Chandrasekhar, Spitzer, Alfven, 
Houtermanns, Atkinson ...) realized the presence and importance of 
ionized gases in outer space, as well as the role they play in nuclear 
fusion reactions in star nuclei, increasing the relevance of Plasma 
Physics on understanding nature (as matter in stars is mainly in the 
plasma state, it is said that probably over 99% of matter in the whole 
Universe is in the plasma state). 

I hope this answer had been what you expected. Almost all the information 
I used for writting it has been extracted from the introduction to the 
book ''Plasma Physics'' by J.G. Linhart (North-Holland, Amsterdam - 
1960). There are mountains of books devoted to the study of Plasma 
Physics. Almost all I know are specialized books written for physicists, 
so could be very hard to read.

To search for more information on Plasma Physics and related topics you 
better surf and visit the links listed here (most of them treat Nuclear 
Fusion as well as Plasma Physics, as Nuclear Fusion is the main field of 
application of plasmas being studied nowadays):

* http://fusion.gat.com/PlasmaOutreach/  
(Various USA Universities and Laboratories have developed a program 
called 'Plasma Sciences and Technologies Education Outreach' with the aim 
of getting ready a place to learn about Plasma Physics and Nuclear 
Fusion).  

* http://FusEdWeb.pppl.gov/ 
(Princeton University Plasma Physics Laboratory)

* http://www-istp.gsfc.nasa.gov/Education/whplasma.html http://astro-2.msfc.nasa.gov/academy/space/PLASMA_PHY.HTML
(NASA's pages on Plasma Physics History and Theory)

Enjoy... and keep on thinking.