... Chapter 1 • Introduction 1.1 A gas at 20°C may be rarefied if it contains less than 10 12 molecules ... for estimating the mean free path of a perfect gas is: 1.26 1.26 (RT) p (RT) µµ ρ ==√ √ l (1) Chapter 1 Introduction 3 where the latter form follows from the ideal-gas law, = p/RT. What are ... ⋅ === C1.00≈ The data show that C = 1, or σ = My/I, our old friend from strength of materials. Chapter 1 Introduction 5 1.9 The dimensionless Galileo number, Ga, expresses the ratio of gravitational...