Regnault&rsquos Condenser Hygrometer
Fig. 78.
(Fig. 78) consists of a tube, C, made of silver, very thin, and perfectly polished; the tube is larger at one end than the other, the large part being 1·8 inches in depth, by 0·8 in diameter; this is fitted tightly to a brass stand, B, with a telescopic arrangement for adjusting when making an observation.
The tube, C, has a sma
A delicate thermometer, D, is inserted through a cork, or India-rubber washer, at the open end of the tube, C, the bulb of which descends to the centre of its largest part.
G is an attached thermometer for taking the temperature of the air, and F is a bottle containing ether.
To use the Condenser Hygrometer, a sufficient quantity of ether is poured into the silver tube to cover the thermometer bulb: on allowing air to pass bubble by bubble through the ether, by breathing in the tube, E, an uniform temperature will be obtained; if the ether continues to be agitated, by breathing briskly through the tube a rapid reduction of temperature will be the result; at the moment the ether is cooled down to the dew-point temperature, the external surface of that portion of the silver tube containing ether will become covered with a coating of moisture, and the degree shown by the thermometer at that instant will be the temperature of the dew-point.
This form of hygrometer, for ascertaining by direct observation the dew-point, is so superior to Daniell’s, both from its being more certain in its indications and economical in use, that Messrs. Negretti and Zambra have been induced to modify it, and reduce its price to little more than that of a good Daniell’s Hygrometer.
Fig. 79.
Temperature of Evaporation.—When the air is not saturated with vapour, evaporation is going on with more or less activity, according as the temperature is high or low, rising or falling. Now vapour cannot be formed without an expenditure of heat; as we invariably find that the process of evaporation lowers the temperature of the liquid from which the vapour is produced, and, by communication, that of contiguous substances also. Thus the emigrant, crossing the line under the scorching influence of the vertical sun, wraps a wet towel round his can of water, swings it in the breeze, to evaporate the moisture of the towel, and obtains a glass of cool water. So also, European residents in India, during the hot season, spread out mats in their apartments, and keep them wet, in order that the evaporation may cool the air. This principle has been applied, for the purpose of ascertaining the hygrometric condition of the air, in the instrument known as Mason’s hygrometer, or psychrometer, which is now in general use, from its simplicity, accuracy, and ease of observing.
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