The English scientist John Dalton (1766-1844) made the observation which has come to be known as "Dalton's Law" and which forms the basis for decompression theory.  In simplified form, Dalton's Law states,

The pressure exerted by a mixture of gases is equal to the sum of the pressures of each of the different gases making up the mixture--each gas acting as if it alone were present and occupied the total volume.




Partial Pressure