* Torricell and Boyle. l. 128. The pressure of the atmosphere was discovered by Torricelli, a disciple of Galileo, who had previously found that the air had weight. Dr. Hook and M. Du Hamel ascribe the invention of the air-pump to Mr Boyle, who however confesses he had some hints concerning its construction from De Guerick. The vacancy at the summit of the barometer is termed the Torricellian vacuum, and the exhausted receiver of an air pump the Boylean vacuum, in honour of these two philosophers.
The mist and descending dew which appear at first exhausting the receiver of an air-pump, are explained in the Phil. Trans. Vol. LXXVIII. from the cold produced by the expansion of air. For a thermometer placed in the receiver sinks some degrees, and in a very little time, as soon as a sufficient quantity of heat can be acquired from the surrounding bodies, the dew becomes again taken up. See additional notes, No. VII. Mr. Saussure observed on placing his hygrometer in a receiver of an air-pump, that though on beginning to exhaust it the air became misty, and parted with its moisture, yet the hair of his hygrometer contracted, and the instrument pointed to greater dryness. This unexpected occurrence is explained by M. Monge (Annales de Chymie, Tom. V.) to depend on the want of the usual pressure of the atmosphere to force the aqueous particles into the pores of the hair; and M. Saussure supposes, that his vesicular vapour requires more time to be redissolved, than is necessary to dry the hair of his thermometer. Essais sur l'Hygrom. p. 226. but I suspect there is a less hypothetical way of understanding it; when a colder body is brought into warm and moist air, (as a bottle of spring-water for instance,) a steam is quickly collected on its surface; the contrary occurs when a warmer body is brought into cold and damp air, it continues free from dew so long as it continues warm; for it warms the atmosphere around it, and renders it capable of receiving instead of parting with moisture. The moment the air becomes rarefied in the receiver of the air-pump it becomes colder, as appears by the thermometer, and deposits its vapour; but the hair of Mr. Saussure's hygrometer is now warmer than the air in which it is immersed, and in consequence becomes dryer than before, by warming the air which immediately surrounds it, a part of its moisture evaporating along with its heat.
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