The hatching of the vipers' eggs?

The following is from Spurgeon's sermon,
"War with Amaklek" No. 712. Exodus 17:8.

It is just where we feel most safe
that we should be most cautious.

Many people need not be much afraid of being
led into drunkenness and blasphemy, for we
are not likely to give way to these grosser evils.

But we have far more reason to watch against
worldliness and pride, for these are enemies
which select the godly as their special object
of attack.

Beware of your virtues, Christian, for these,
when exaggerated, become your vices.

Beware of the good things in which you boast,
for they may furnish heat for the hatching of
the vipers' eggs
of pride and self-satisfaction!

With what vigilance, what holy diligence
must you and I watch against the windings
and twistings of the old serpent, who will, if
possible, bite our heels or worm his venom
into our hearts!