That's a good point. Perhaps the result of that argon/nitrogen comparison was a coincidence, or perhaps heat capacity doesn't much affect thermal conductivity.

That's a good point. Perhaps the result of that argon/nitrogen comparison was a coincidence, or perhaps heat capacity doesn't much affect thermal conductivity. Here's an argument that heat capacity shouldn't affect thermal conductivity in a solid: imagine a block of material with a smooth temperature gradient through it and the temperature at any point remaining constant, so there is heat flowing through it. Now imagine you have some tiny beads of some other material with a super-high heat capacity. Distribute those beads throughout the block of material, preheating each bead to the correct local temperature before inserting it. Then intuitively it seems that neither any local temperature nor the rate of flow of heat through the block should change much. Yet the heat capacity of the new composite material is significantly higher than that of the original material. Does that argument make sense? I did physics at school but not since then!