From the preamble of the edition published by PG:

  Note that this version of Thesaurus-1911 has been supplemented with over 
  1,000 words not present in the original 1911 edition, but many modern words 
  are still missing. About 1500 verbs (out of 6500) which can be found in an 
  80,000-word spell-checker are absent from this work. The deficiency of nouns 
  is probably much worse, especially on technical topics. Of 40,000 unique 
  words contained in the original text, 12,000 are not recognized by a 
  spell-checker. Most of these are foreign words (primarily Latin), and many 
  are obsolete.