A young man holding his hand to his heart, emanating insensible perspiration. Colour stipple engraving by J. Pass, 1794, after E. Sibly.

"In the annexed copper-plate engraving, I have endeavoured to shew the manner in which the insensible perspiration issues from the pores of the body, which can only be discerned by means of a lens; being of so volatile and subtle a quality, that it passes through our garments with the utmost ease, particularly if woollen; and it even ascends through the bed-clothes like a mist, in the greatest abundance when we are asleep, and the animal functions at rest. In this manner nature, from all casual obstructions, endeavours to relieve herself; and so long as diseases are recent, and of a mild tendency, they are usually carried off by this means, without requiring any aid from medicine"--Sibly, op. cit., pp. 126-127