employ vs slave labour

employ

noun
  • The state of being an employee; employment. 

verb
  • To hire (somebody for work or a job). 

  • To use (somebody for a job, or something for a task). 

  • To make busy. 

slave labour

noun
  • The employment of workers for meagre pay to do tedious tasks. 

  • Using slaves to do work 

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