material file definition v3.1
29 Jan 1996 MJF

Both the xspect routines and the skeptic routines take their
definition of a material from material files located in _materials
which is usually set up as a part of a link in the initial portion
of a script. The files are to have the following form.

line 1:  no. of elements in the material (limited to 10)
line 2:  atomic number (Z) of each element in the material
line 3:  either:  a) No. of atoms per molecule (if integer entries)
		  b) weight fraction per element (if fractions < 0)
		  (1.0 if only one element is present)
line 4: density.
line 5: C or G : for condensed or gas
line 6: Pressure for G in mmHg (only for G in line 5)

The Chem/Phys handbook notes for the ideal gas law , R = (PV)/(nT), that
R = 62,400 when P is in mmHg, V is in cm3 , n is in grams, and T is in Kelvin.
We will need to keep this is mind when entering the fractions and density
for the gases.

Note: the xspect routines currently limit the number of different elements
      that can be present in a material to 10.