[…] I have observed in grinding of glasses […] sometimes the convex surface of one body being ground upon the concave surface of another, the two surfaces will happen to be so closely and exactly fitted to one another, (their immediate contact in all their parts, or at least in innumerable of them, hindering the intercurrence of the air) that a man is not able without breaking one or both of them to pull them directly asunder […]
Source: wiktionary