  We have explored the relationship between hard X-ray (HXR) emissions and
Doppler velocities caused by the chromospheric evaporation in two X1.6 class
solar flares on 2014 September 10 and October 22, respectively. Both events
display double ribbons and Interface Region Imaging Spectrograph (IRIS) slit is
fixed on one of their ribbons from the flare onset. The explosive evaporations
are detected in these two flares. The coronal line of Fe XXI 1354.09 A shows
blue shifts, but chromospheric line of C I 1354.29 A shows red shifts during
the impulsive phase. The chromospheric evaporation tends to appear at the front
of flare ribbon. Both Fe XXI and C I display their Doppler velocities with a
`increase-peak-decrease' pattern which is well related to the `rising-maximum-
decay' phase of HXR emissions. Such anti-correlation between HXR emissions and
Fe XXI Doppler shifts, and correlation with C I Doppler shifts indicate the
electron-driven evaporation in these two flares.
