Unfortunately, thallium was not discovered by my great-grandfather Ezra Thall but by British chemist/physicist William Crookes.  At the tender age of 25, Crookes inherited a fortune from his father and proceeded to devote the rest of his life pursuing scientific research (Ezra Thall would have pursued other interests had he inherited the fortune).  Crookes is best known for work on electrical discharges through rarefied air that led him to observe the dark space around the cathode (appropriately called Crookes dark space).  In the process he developed the Crookes tube or cathode ray tube.  Crookes was knighted in 1897 and hence the "Sir."   Physics Nobel Laureate J.J. Thomson modified Crookes' tube and in 1897 discovered the electron but that's another story.  Back to thallium and spectrum analysis.
 

Spectrum Analysis
Around 1859 at the University of Heidelberg in Germany, Robert Bunsen (1811-1899) and Gustav Kirchhoff (1824-1887) developed what has become known as spectrum analysis (use of light or electromagnetic radiation to analyze substances). Gustav Kirchhoff had the brilliant insight to use a prism to separate the light of heated substances into constituent rays, thus the fledgling science of spectroscopy, which would develop into a vital tool for chemical analysis, was born. Bunsen and Kirchhoff developed the spectroscope shown below from a prism, cigar box, and two ends of old telescopes.

Using this instrument, Bunsen/Kirchhoff were able to identify elements since each element has its own unique atomic spectrum as illustrated for hydrogen, helium, lithium, and mercury:

Bunsen/Kirchhoff perfected the technique by noting that the lines became more distinct at higher  temperatures and lower luminescence of the flame.  The instrument proved to be of tremendous importance not only in chemical analysis but in the discovery of new elements.  In 1861 Bunsen/Kirchhoff announced the discovery of cesium (Latin "caesium" meaning sky blue) in the following passage:
      "Supported by unambiguous results of the spectral-analytical method, we believe
       we can state rightnow that there is a fourth metal in the alkali group besides
      potassium, sodium, and lithium, and ithas a simple characteristic spectrum like
      lithium; a metal that shows only two lines in our apparatus: a faint blue one,
      almost coinciding with strontium and another blue one a little further to the violet
      end of the spectrum and as strong and as clearly defined as the lithium line."
A few months after the cesium discovery, Bunsen/Kirchhoff discovered another new alkali metal.  This element was named rubidium from the Latin "rubidus" meaning darkest red color.

Bunsen/Kirchhoff published Chemical Analysis by Observation of Spectra and undoubtedly Crookes was aware of their work.  So when Crookes (1861) noted a bright green spectral line resulting from heating selenium minerals, he realized immediately that the line represented a new element.  Crookes, following the naming trend set by Bunsen/Kirchhoff, called the element thallium from the Greek word "thallos" meaning green shoot.  After isolating thallium, Crookes studied its properties and determined the atomic weight.
Eventually the puzzle of atomic spectra was explained by Niels Bohr.  When heated, electrons in atoms jump to a higher energy level and upon returning to their normal energy level, emit energy in the form of electromagnetic radiation (light). The atomic spectra wavelengthsprovided crucial data for determining atomic structure.

Properties
Thallium is the heaviest of the Group IIIa elements and usually takes on charge of +1 (Tl⁺¹ resembles the heavier alkali metal ions).  Tl⁺³ compounds are readily reduced to Tl⁺¹.
Tl°    [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹
Tl⁺¹   [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p°
Tl⁺³   [Xe] 6s° 4f¹⁴ 5d¹⁰ 6p°

Like lead, thallium is a soft, low-melting element of low tensile strength.  Freshly cut thallium has a metallic luster that dulls to bluish gray upon exposure to air.  Thallium dissolves slowly in hydrochloric acid and dilute sulfuric acid and rapidly in nitric acid.

Melting Point	303.5°C
Boiling Point	1457°C
Density	11.85 g/mL
Atomic Radius	2.08 A
Heat of Vaporization	164 kJ/mol
Ionization Potential	6.11 V
Ionization Potential	4.14 kJ/mol
Electronegativity	2.04
Specific Heat	0.13 J/g/K

With its position in the periodic table between poisonous heavy metals mercury and lead, it is not surprising that thallium compounds are toxic.  Thallium poisoning, which may be fatal, causes nervous and gastrointestinal disorders and rapid loss of hair.  Natural thallium consists almost entirely of a mixture of two stable isotopes:

    ²⁰³Tl  (29.50%)          ²⁰⁵Tl  (70.50%)

Traces of short-lived isotopes occur as decay products in three natural radioactive disintegration series:
Uranium series produces  ²⁰⁶Tl  &  ²¹⁰Tl
Thorium series produces  ²⁰⁸Tl
Actinium series produces  ²⁰⁷Tl
 

Isotope	Half Life
Tl-200	1.08 days
Tl-201	3.03 days
Tl-202	12.23 days
Tl-203	stable
Tl-204	3.78 years
Tl-205	stable
Tl-206	4.2 minutes
Tl-207	4.77 minutes
Tl-208	3.05 minutes
Tl-209	2.2 minutes
Tl-210	1.3 minutes

Uses
Uses for thallium include thallium-201 stress test, rodenticides, photocells, infrared detectors, low melting glasses, and treatment for skin infections