The tube has a 10cm capillary in the center for observing light wavelengths and is straight form with side electrodes for conductivity. The tube measures 26cm and is made of glass for transparency and resistance to heat. Glass is an inert brittle solid, heat-resistant, and typically transparent. It is relatively easily cut and shaped. 18 sets of Spectrum tubes - 1. Carbon Dioxide 2. Nitrogen 3. Oxygen 4. Neon 5. Argon 6. Chlorine 7. Ammonia 8. Iodine 9. Sulphur 10. Krypton 11. Cadmium 12. Mercury Vapor 13. Alcohol Vapor 14. Water Vapor 15. Hydrogen 16. Sodium 17. Helium 18. Air