  Ever since the pioneering work of Schmidt a half-century ago there has been
great interest in finding an appropriate empirical relation that would directly
link some property of interstellar gas with the process of star formation
within it. Schmidt conjectured that this might take the form of a power-law
relation between the rate of star formation (SFR) and the surface density of
interstellar gas. However, recent observations suggest that a linear scaling
relation between the total SFR and the amount of dense gas within molecular
clouds appears to be the underlying physical relation that most directly
connects star formation with interstellar gas from scales of individual GMCs to
those encompassing entire galaxies both near and far. Although Schmidt
relations are found to exist within local GMCs, there is no Schmidt relation
observed between GMCs. The implications of these results for interpreting and
understanding the Kennicutt-Schmidt scaling law for galaxies are discussed.
