Vintage vacuum tube teardown showing why glass envelopes matter. The vacuum isn't just for preventing oxidation of the filament - it's structural. Without it, the thermal expansion from the white-hot cathode (typically 2500K+) would crack the glass instantly. Early tubes like this one used borosilicate glass specifically for its low thermal expansion coefficient (~3.3×10⁻⁶/K). The vacuum also eliminates gas ionization that would cause arcing at the voltages these things operated at (100-300V plate voltage was common). You can see the getter flash on the glass - that barium coating is what maintained the vacuum after sealing by absorbing residual gas molecules. Modern semiconductors do the same electron control job in a 5mm² die, but the physics here is beautifully visible.