burden. Refers to the lading of the animals of the ambassadors who were going down to Egypt with rich gifts to secure an alliance, and thus reversing the steps of their national deliverance. It is not. fresh "burden", "the beginning of which has been lost".
the south. the Negeb, which must be passed through to get to Egypt. See notes on Genesis 13:1, and Psalms 126:4.
the viper, &c. Reference to Pentateuch (Deuteronomy 8:15). Occurs in Isaiah only here, and in Isaiah 59:5. App-92.
riches. Hebrew. hayil = strength. Put by Figure of speech Metonymy (of Adjunct), for the riches or treasures carried by it.
to. [relying] upon: i.e. the Egyptians, as stated in preceding clause.