The whole amount of heat which is consumed in the conversion of water into vapour is not used in overcoming the cohesion--that is, in internal accomplished in the liquid. A part of this heat is employed in moving the aqueous particles; in fact, aqueous vapour at 100° occupies a volume 1,659 times greater than that of water (at the ordinary pressure), consequently a portion of the heat or work is employed in lifting the aqueous particles, in overcoming pressure, or in external work, which may be usefully employed, and which is so employed in steam engines. In order to determine this work, let us consider the variation of the maximum _pressure_ or _vapour tension of steam_ at different temperatures. The observations of Regnault in this respect, as on those preceding, deserve special attention from their comprehensiveness and accuracy. The pressure or tension of aqueous vapour at various temperatures is given in the adjoining table, and is expressed in millimetres of the barometric column reduced to 0°. +------------+---------+-------------+----------+ |Temperature | Tension | Temperature | Tension | +------------+---------+-------------+----------+ | -20° | 0·9 | 70° | 233·3 | | -10° | 2·1 | 90° | 525·4 | | 0° | 4·6 | 100° | 760·0 | | +10° | 9·1 | 105° | 906·4 | | 15° | 12·7 | 110° | 1075·4 | | 20° | 17·4 | 115° | 1269·4 | | 25° | 23·5 | 120° | 1491·3 | | 30° | 31·5 | 150° | 3581·0 | | 50° | 92·0 | 200° | 11689·0 | +------------+---------+-------------+----------+