harden vs weaken

harden

verb
  • To make something hard or harder (tough, resistant to pressure). 

  • To become or make (a person or thing) resistant or less sensitive. 

  • To become or make (a consonant) more fortis; to (cause to) undergo fortition. 

  • To strengthen. 

  • To modify (a website or other system) to make it resistant to malicious attacks. 

  • To unpalatalize or velarize. 

  • To become hard (tough, resistant to pressure). 

weaken

verb
  • To make weaker or less strong. 

  • To become weaker or less strong. 

How often have the words harden and weaken occurred in a corpus of books? (source: Google Ngram Viewer )