What fluid is contained within the compass case of a Cessna 172?

The fluid contained within the compass case of a Cessna 172 is kerosene. This specific fluid is used because of its low viscosity and relatively stable temperature characteristics, which help to ensure accurate readings across different temperatures and conditions. Kerosene has a lower freezing point than many other fluids, making it an ideal choice for aviation applications where temperature fluctuations can occur.

While other fluids like alcohol and water have been used in various applications, they do not provide the same level of performance and reliability required for aviation compasses. Alcohol can evaporate or expand significantly with temperature changes, and water can freeze or cause corrosion, which would negatively impact the functionality of the compass. Thus, kerosene is preferred for its stability and performance in the harsh environments experienced during flight.