In March 1823, English experimentalist and chemist Michael Faraday (1791–1867) at the Royal Institution in London published a breakthrough paper in the Philosophical Transactions of the Royal Society titled “On Fluid Chlorine”. Faraday achieved what 18th-century science had deemed impossible: the controlled liquefaction of a supposed “permanent gas”—chlorine ($\text{Cl}_2$).

By heating chlorine hydrate inside a sealed, heavy-walled glass V-tube while cooling the opposite end in an ice bath, Faraday generated high internal pressure that forced the gaseous chlorine to condense into a bright yellow liquid. Faraday’s discovery proved that gases are simply the vapor state of volatile liquids, shattering early thermodynamic dogma and launching the cryogenic science that eventually unlocked superfluidity, superconductivity, and quantum computer dilution refrigeration.

The Sealed V-Tube Apparatus & Discovery

Prior to 1823, natural philosophers divided gaseous matter into two distinct categories: condensable vapors (such as steam) and permanent gases (such as oxygen, nitrogen, hydrogen, and chlorine), which were believed to be intrinsically incapable of existing in a liquid state regardless of pressure or temperature.

The Accidental Experiment

While investigating green crystals of chlorine hydrate ($\text{Cl}_2 \cdot 10\text{H}_2\text{O}$), Sir Humphry Davy suggested Faraday heat the crystals inside a sealed glass tube:

  1. Faraday placed chlorine hydrate crystals in the sealed long leg of a heavy bent glass tube (a “V-tube”).
  2. Heating the crystals caused them to decompose, releasing dense chlorine gas ($\text{Cl}_2$) and water: $$\text{Cl}_2 \cdot 10\text{H}_2\text{O}(s) \xrightarrow{\Delta} \text{Cl}_2(g) + 10\text{H}_2\text{O}(l)$$
  3. As gas evolved inside the closed, rigid glass volume, the internal pressure mounted rapidly to approximately $5\,\text{to}\,6\,\text{atmospheres}$ ($500\,\text{kPa}$).
  4. At the cool, unheated end of the tube (immersed in ice water), a heavy, bright oily yellow liquid accumulated—pure liquid chlorine!

Faraday immediately recognized the general applicability of his technique. In subsequent weeks, he successfully liquefied sulfur dioxide ($\text{SO}_2$), hydrogen sulfide ($\text{H}_2\text{S}$), ammonia ($\text{NH}_3$), carbon dioxide ($\text{CO}_2$), and nitrous oxide ($\text{N}_2\text{O}$).

Thermodynamics of Phase Transitions & Critical Phenomena

Faraday’s work demonstrated that phase transitions between gas and liquid are governed by pressure ($P$) and temperature ($T$), providing the empirical foundation for 19th-century phase thermodynamics:

1. Clausius-Clapeyron Equation

The equilibrium phase boundary separating liquid and gas phases on a $P-T$ phase diagram is governed by the Clausius-Clapeyron equation:

$$\frac{dP}{dT} = \frac{\Delta H_{\text{vap}}}{T \Delta V}$$

Where $\Delta H_{\text{vap}}$ is the latent heat of vaporization and $\Delta V = V_{\text{gas}} - V_{\text{liquid}}$ is the volume change during the phase transition. Increasing pressure $P$ elevates the condensation temperature $T$, allowing chlorine to condense at $0^\circ\text{C}$ under $4.9\,\text{atm}$ of pressure.

2. Van der Waals Equation of State (1873)

Johannes Diderik van der Waals explained why gases require pressure and cooling to liquefy by modifying the ideal gas law to account for finite molecular volume ($b$) and attractive intermolecular forces ($a$):

$$\left( P + \frac{a n^2}{V^2} \right) (V - n b) = n R T$$

Van der Waals derived the critical temperature $T_c$ above which no amount of pressure can liquefy a gas:

$$T_c = \frac{8a}{27 R b}$$

Chlorine has a high critical temperature ($T_c = 417\,\text{K} = 144^\circ\text{C}$), which is why Faraday easily liquefied it at room temperature under pressure. Oxygen ($T_c = 154.6\,\text{K}$) and Hydrogen ($T_c = 33.2\,\text{K}$) required deep cryogenic cooling beyond Faraday’s 1823 capabilities.

The Cryogenic Cascade to Absolute Zero

Faraday’s liquefaction breakthrough triggered an international scientific race to liquefy all known gases, steadily lowering laboratory temperatures toward absolute zero ($0\,\text{K}$):

  • 1877 (Pictet & Cailletet): First liquid oxygen ($\text{O}_2$) and liquid nitrogen ($\text{N}_2$) sprays ($77\,\text{K}$).

  • 1898 (James Dewar): Liquefied hydrogen ($\text{H}_2$) at $20.28\,\text{K}$ using regenerative cooling and invented the vacuum-insulated Dewar flask.

  • 1908 (Heike Kamerlingh Onnes): Liquefied Helium-4 ($^4\text{He}$) at $4.2\,\text{K}$ at Leiden University, opening the realm of quantum liquids and discovering Superconductivity in mercury in 1911.

Bridge to Quantum Liquids & Cryogenic Quantum Computers

Faraday’s pioneer gas liquefaction lies at the heart of modern quantum hardware and low-temperature physics:

1. Superfluidity & Bose-Einstein Condensation

When liquid $^4\text{He}$ is cooled below the lambda point ($T_\lambda = 2.17\,\text{K}$), it undergoes a quantum phase transition into a superfluid—a quantum fluid with zero viscosity, infinite thermal conductivity, and quantized vortices described by a single macroscopic wave function $\Psi(\mathbf{r}) = \sqrt{\rho} e^{i\theta}$.

2. Dilution Refrigeration for Quantum Hardware

Modern superconducting quantum processors (such as IBM Quantum Heron and Google Sycamore transmons) and topological qubit platforms must operate at cryogenic temperatures near absolute zero ($10\,\text{mK} = 0.01\,\text{K}$) to prevent thermal noise from destroying quantum superposition and entanglement.

These ultra-low temperatures are achieved using $^3\text{He}/^4\text{He}$ dilution refrigerators—a direct technological descendant of Michael Faraday’s 1823 gas liquefaction experiments.

Key Takeaways

  • Year: 1823

  • Key Figure: Michael Faraday (English Experimentalist & Physicist)

  • Core Discovery: Achieved the first controlled liquefaction of chlorine gas ($\text{Cl}_2$) using a sealed glass V-tube under self-generated pressure.

  • Conceptual Impact: Shattered the concept of “permanent gases,” proving that all gases are vapors of volatile liquids governed by $P-T$ phase boundaries.

  • Thermodynamic Basis: Underpins the Clausius-Clapeyron equation, van der Waals equation of state, and critical temperature ($T_c$).

  • Quantum Relevance: Initiated the cryogenic physics line that led to liquid helium ($4.2\,\text{K}$), superfluidity, superconductivity, and dilution refrigerators for quantum processors.