statement that the weight of a gas dissolved by a liquid is proportional to the pressure of the gas upon the liquid. The law, which was first formulated in 1803 by the English physician and chemist William Henry (Henry, William), holds only for dilute solutions (solution) and low gas pressures.

In a very dilute solution, a solute molecule will (with rare exceptions) have only solvent molecules as near neighbours, and the probability of escape of a particular solute molecule into the gas phase is expected to be independent of the total concentration of solute molecules. In this case the rate of escape of solute molecules will be proportional to their concentration in the solution, and solute will accumulate in the gas until the return rate is equal to the rate of escape. With a very dilute gas this return rate will be proportional to the partial pressure of solute. Thus, we expect that, for a solution very dilute in solute, in equilibrium with a gas at very low pressure, the gas pressure will be proportional to the amount of dissolved gas—the relation known as Henry's law. While the above argument is to be considered only suggestive, Henry's law is found experimentally to hold for all dilute solutions in which the molecular species is the same in the solution as in the gas. The most conspicuous apparent exception is the class of electrolytic solutions.