We selected a sample of 859 flares observed in hard X-rays (HXRs) by BATSE and in soft X-rays (SXRs) by GOES to study the timing of the SXR and HXR emission. Each sample event presents a simple light curve; i.e., the HXR has only one short time peak and the SXR seems to be a response to the pulse HXR emission. The statistical study shows that the peak time differences between the SXR and HXR emissions conform to a decay exponential function. This distribution, being similar to that for the flaring loop size discovered previously, suggests that the flaring loop size is the reason for the peak time difference between the SXR and HXR emissions. The longer the flaring loop is, the longer the time for chromospheric evaporation reaching the loop top is, i.e., the later the SXR peaks. Our result here provides a support for the traditional chromospheric evaporation model. The fact that the SXR can increase for some time after the end of HXR emission does not seem to be in contradiction with the Neupert effect, if one considers that the electron-driven evaporation needs some time to reach the loop top. Further hydrodynamic modeling is obviously necessary to explain the observations.
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