"William Thomson... in March 1851... regarded this result as a proof of the improbability of the above assumption which I had employed. Since then, however, he and J. P. Joule have together undertaken to test experimentally the accuracy... [and] have... shown... with... permanent gases, atmospheric air and hydrogen, the assumption is so nearly true... deviations from exactitude may be disregarded. With [non-permanent gas,] ... deviations were greater... in... accordance with...[my] remark... that the latter would probably be... be accurate for each gas in the same measure as Mariotte's and Gay-Lussac's laws were applicable... Thomson now calculates the volumes of saturated vapours in the same manner as myself."