ther, and the mofl: obvious folution is, that the rain has penetrated through thefe walls. The following fads, however, are fufR- cient to prove the error of this folution.
I. This dew upon the walls appears when there is no rain, if the ftate of the atmof- phere be moift. 2. It appears on the par- tition as well as the exterior walls. 3. So alfo on pavements of brick or ftone. 4.
It is more copious in proportion as the walls are thicker; the reverfe of which ought to be the cafe, if this hypothefis were juft. If cold water be poured into a veffel of ftone, or glafs, a dew forms inftantly on the outfide : but if it be poured into a vejfTel of wood, there is no fuch appearance. It is not fuppofed, in the firft cafe, that the water has exuded through the glafs, but that it is precipitated from the circumambient air; as the humid par- ticles of vapour, paifing from the boiler of an alembic through its refrigerant, are pre- cipitated from the air, in which they are fufpended, on the internal furface of the refrigerant. Walls of brick or ftone a6t as the refrigerant in this inftance. They are fufRciently cold to condenfe and preci- pitate the moifture fufpended in the air of the room, when it is heavily charged there-