We re-examine the X–ray luminosity–temperature relation using a nearly homogeneous data set of 24 clusters selected for statistically accurate temperature measurements and absence of strong cooling flows .
The data exhibit a remarkably tight power–law relation between bolometric luminosity and temperature with a slope 2.88 \pm 0.15 .
With reasonable assumptions regarding cluster structure , we infer an upper limit on fractional variations in the intracluster gas fraction \langle ( \delta \hbox { $f _ { gas } $ } / \hbox { $f _ { gas } $ } ) ^ { 2 } \rangle ^ { 1 / 2 } \leq 15 \% .
A strictly homogeneous GINGA subset of 18 clusters places a more stringent limit of 9 \% .

Imaging data from the literature are employed to determine absolute values of f _ { gas } within spheres encompassing density contrasts \delta _ { c } = 500 and 200 with respect to the critical density .
Comparing binding mass estimates based on the virial theorem ( VT ) and the hydrostatic , \beta –model ( BM ) , we find a temperature–dependent discrepancy in f _ { gas } between the two methods caused by systematic variation of the outer slope parameter \beta with temperature .
Mean values ( for { H _ { o } } = 50 { km s ^ { -1 } Mpc ^ { -1 } } ) range from \hbox { $ { \bar { f } } _ { gas } $ } = 0.10 for cool ( T < 4 { keV } ) clusters using the VT at \hbox { $ \delta _ { c } $ } = 500 to 0.22 for hot ( T > 4 { keV } ) clusters using the BM at \hbox { $ \delta _ { c } $ } = 200 .
There is evidence that cool clusters have a lower mean gas fraction than hot clusters , but it is not possible to assess the statistical significance of this effect in the present dataset .
The T dependence of the ICM density structure , coupled with the increase of the gas fraction with T in the VT approach , explains the steepening of the L _ { X } – T relation .

The small variation about the mean gas fraction within this majority sub–population of clusters presents an important constraint for theories of galaxy formation and supports arguments against an Einstein–deSitter universe based on the population mean gas fraction and conventional , primordial nucleosynthesis .
The apparent trend of lower gas fractions and more extended atmospheres in low temperature systems are consistent with expectations of models incorporating the effects of galactic winds on the intracluster medium .