‘Proof’ by Hal Clement
As the introduction makes clear, Clement is the proto-typical hard SF writer and so this story proves. There is a lot of this sort of thing:
since gaseous friction is independent of change in density, the high pressures of the regions being penetrated wopuld be no bar to high speed flight. Unfortunately, the viscosity of a gas does increase directly as the square root of its temperature; at the lower levels of the Sun, travel would be slow. (p. 95)
As this demonstrates, in science fiction hardness often goes hand in hand with that other physical property: dryness. At the same time, it is told in the genial style of a club story:
The other members of the crew shifted slightly in their cradles; a ripple of interest passed through them, for they had heard rumours and vague tales of Kron’s time in the space carrier fleets. The Sirian settled himself more comfortably; Kron dimmed the central globe of radiance a trifle, for the outside temperature was now considerably higher, and began the tale. (p. 95)
No, not much to interest a reader like me here.