In a paper lately communicated to the Chemical Society, I endeavoured to show that an exact examination of the rate at which water was expelled from hydrated substances at any constant temperature, might teach us much as to the constitution of these bodies. One of the bodies examined was the mineral Gypsum, which has been found in three different states—first, in its most usual form as Ca SO4, 2 H2O, or as it should be written 2 Ca SO4, 4 H2O as iu selenite ; second as 2 Ca SO4, H2O in which three of the molecules of water have disappeared, as in certain un-namcd forms of Anhydrite ; and third as Ca SO4 as in Anhydrite. Now these three can also be obtained by artificial means ; the second being found in steam boilers where the water is evaporated under pressure; so I made several attempts to obtain the other two possible hydrates, 2 Ca SO4 3 H2O, and 2 Ca SO4, 2 H2O by evaporating solutions Under different pressures, but I could obtain no satisfactory results.