Robert Boyle (1627–1691)

From The Cause of Attraction by Suction

AMONG the more familiar phenomena of the Machina Boyliana, as they now call it, none leaves so much scruple in the minds of some sorts of men as this, that when one’s finger is laid close upon the orifice of the little pipe by which the air is wont to pass from the receiver into the exhausted cylinder, the pulp of the finger is made to enter a good way into the cavity of the pipe, which doth not happen without a considerable sense of pain in the lower part of the finger. For most of those that are strangers to hydrostatics, especially if they be prepossessed with the opinions generally received both in the peripatetic and other schools, persuade themselves that they feel the newly mentioned and painful protuberance of the pulp of the finger to be effected, not by pressure, as we would have it, but distinctly by attraction.

To this we are wont to answer that, common air being a body not devoid of weight, the phenomenon is clearly explicable by the pressure of it; for, when the finger is first laid upon the orifice of the pipe, no pain nor swelling is produced, because the air which is in the pipe presses as well against that part of the finger which covereth the orifice, as the ambient air doth against the other parts of the same finger. But when, by pumping, the air in the pipe, or the most part of it, is made to pass out of the pipe into the exhausted cylinder, then there is nothing left in the pipe whose pressure can anything near countervail the undiminished pressure of the external air on the other parts of the finger; and consequently, that air thrusts the most yielding and fleshy part of the finger, which is the pulp, into that place where its pressure is unresisted, that is, into the cavity of the pipe, where this forcible intrusion causeth a pain in those tender parts of the finger.

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