Henry’s law can even be applied to cracking knuckles. Bone ends in joints are separated by a cavity filled with synovial fluid in which gasses are dissolved. This mixture of gases is estimated to be made of ~80% carbon dioxide and the remaining 20% to consist of nitrogen and oxygen [6]. When, the volume of this cavity is changed suddenly (i.e. by stretching ‘cracking’ the joint) the pressure in the cavity decreases. According to Henry’s Law, the decrease in pressure results in some of the dissolved gas leaving the solution, forming a bubble - this process is called cavitation (and the bubble formed is termed a ‘vapour cavity’). Cavitation produces a noise, via vibrations caused from the bubble disrupting the synovial fluid, giving the distinct ‘cracking’ noise [7].
[11] (knuckles...)