Note 61 — Chapter I · ON WATER AND ITS COMPOUNDS

The essence of the matter may he thus represented. A gaseous or easily volatile substance A forms with a certain quantity of water, nH2O, a definite complex compound AnH2O, which is stable up to a temperature t° higher than 100°. At this temperature it is decomposed into two substances, A + H2O. Both boil below t° at the ordinary pressure, and therefore at t° they distil over and re-combine in the receiver. But if a part of the substance AnH2O is decomposed or volatilised, a portion of the undecomposed liquid still remains in the vessel, which can partially dissolve one of the products of decomposition, and that in quantity varying with the pressure and temperature, and therefore the solution at a constant boiling point will have a slightly different composition at different pressures.