Caesium Element Number 55 Cs 

 A small vial of pure Caesium metal, which will melt when held in the hand. If this was to break,  the metal would ignite violently and burn white hot uncontrollably.

 Cs 55 alkali  AW: 132.9054519  D:1.93  MP: 83.19   BP:1240  Mohs: 0.2

Cesium or Caesium is a extremely reactive alkaline metal. Cesium is a silver metal with a low melting point. This element is pyrophoric, and  will ignite and burn with a fierce hot flame if exposed to air, it is very dangerous, and should be handled with extreme caution.

Like Rubidium, Cesium was discovered by Bunsen, and Kirchhoff, and is found in the same minerals. Cesium is used in the "atomic clock" as a "frequency standard". Atomic clocks using Cesium can keep time with accuracy of 1 to 1,000,000,000,000,000 or basically the clock is so accurate it would be off about 1 second every 30 million years. Cesium frequency standards are also used in Internet and cell phone timing, and other electronic applications.

Cesium is used in many tube applications, as getter's for oxygen, PMT tubes, scintillation components. photocells, rectifier, lasers and other optical and tube applications. Cesium is used in the oil industry as a drilling fluid component.

Isotopes of Cesium include Cesium 137 which is  basically a by product of nuclear reactors AKA radioactive waste. Cesium 137 is a hazardous gamma emitter, with about a 30 year half life, it will decay to Ba 137 which is stable. Cs137 has uses in source material for certain equipment  such as X ray, moisture, density, and other measurement devices. Cs137 also is used to irradiate medical equipment, sewer sludge, and other items. Cesium 137 has a place in cancer treatment.

Cesium has been  used for ion engines for space craft. Used with a Tungsten electrode Cesium produce electrons and can thrust a craft in space. This has been replaced with less reactive materials such as Xenon.

 

Cesium Nitrate crystals 

 Barium Element Number 56 Ba

 Barium is a alkaline earth metal, that does not exist in pure form on earth. This element is a soft whitish metal, usually kept under Argon or oil. Barium and most of its compounds are toxic, and should be handled with caution.

The Barium mineral Barite fluoresces for years when exposed to light, and was popular with alchemists and other 'sorcerers". Barium ores are generally dense, Thus the name originates from the Greek word for heavy.

Barium is used as compounds in a few industries, Barium Sulfate enemas are used in gastrointestinal x ray, Barium Peroxide is used as a bleaching agent, and to start railroad track welding compounds. Barium is also used in optics, glass manufacture, green fireworks, and coloring, and as a "getter" for Oxygen removal in vacuum tubes.

Barium was used in the production of pure Oxygen. Barium Oxide reacts with air, and form Barium Peroxide at certain high temperatures.,At higher temperatures the Barium peroxide decomposes and releases pure Oxygen.

My Barium samples are kept under paraffin oil. I have small chunks of the pure metal. This material is toxic, and I handle it with caution

 

Small pure Barium chunks, These pieces resemble rocks and each has an interesting appearance. 

Ba 56 Alkaline earth  AW: 137.33  D:3.51  MP: 1341 BP:3447  Mohs: 1.25 

Barite crystal on Calcite.