ossify vs vitrify

ossify

verb
  • To transform (or cause to transform) from a softer animal substance into bone; particularly the processes of growth in humans and animals. 

  • To become (or cause to become) inflexible and rigid in habits or opinions. 

  • To grow (or cause to grow) formulaic and permanent. 

  • To calcify. 

vitrify

verb
  • To convert into glass or a glass-like substance by heat and fusion. 

  • To be converted into glass, especially through heat. 

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