Elements of chemistry : including the applications of the science in the arts . other (b) is dry. The wet thermometeralways indicates a lower temperature than the dry one,unless when the air is fully saturated with moisture,and no evaporation from the moist bulb takes making an observation, the instrument is generallyplaced, not in absolutely still air, but in an open win-dow where there is a slight draught. The indications of the wet-bulb hygrometer, or psy-chrometer, are discovered by simple inspection. It is, therefore, a problem of thegreatest importance to deduce from them the de
Elements of chemistry : including the applications of the science in the arts . other (b) is dry. The wet thermometeralways indicates a lower temperature than the dry one,unless when the air is fully saturated with moisture,and no evaporation from the moist bulb takes making an observation, the instrument is generallyplaced, not in absolutely still air, but in an open win-dow where there is a slight draught. The indications of the wet-bulb hygrometer, or psy-chrometer, are discovered by simple inspection. It is, therefore, a problem of thegreatest importance to deduce from them the dew point, or the tension of the vapourin the air, by an easy rule. Could this inference be made with certainty, the wet-bulb hygrometer is so commodious that it would supersede all others. I shall place 1 The present and following methods of hygrometry, and all the experimental data required,have lately received a full and critical revision from M. Regnault, of the greatest his Etudes sur lHygrometrie, Annales de Chimie, &c, 1835, 3 scr. t. xv. p. 129. Fig. SPONTANEOUS EVAPORATION. 93 below a formula for this purpose, -which has been used for several years in the northof Europe, and the same as it has been recently 4. The most simple mode of ascertaining the absolute quantity of vapour in theair is to cool the air gradually, and note the degree of temperature at which it beginsto deposit moisture, or ceases to be capable of sustaining the whole quantity of vapourwhich it possesses. The air is saturated with vapour for this particular degree oftemperature, which is called its dew-point. The saturating quantity of vapour forthe degree of temperature indicated may then be learned by reference to a table ofthe tension of the vapour of water at different It is the absolutequantity of vapour which the air at the time of the observation possesses. Thedew-point may be ascertained most accurately by exposing to the air a thin cup ofsilver or tin-p
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