I purchased this book by The Edwal Laboratories, Inc, 3rd Edition, 1947 at a camera store junk box still wrapped. What I find interesting are the tips that are given; and these tips have been around for 51 years or perhaps longer since the 1st Edition was published August 1939.
1. To test the "clearing time" of fixer, Edwal recommends that you soak a piece of negative film 3 to 4 minutes first before placing it into the fixer.
Lots of posts on this subject on APUG: Several this week. Use the film leader or a ~ 1 inch piece cut from a roll (or sheet) of the film. With the room lights on, place the film strip in fresh fixer. With agitation, time how long it takes for the film to become completely transparent (or clear). This is the clearing time for this film/fixer combination. Multiply the clearing time by three for the total fixing time. Ideally, you should perform this clearing time test before each fixer use.
When the clearing time doubles the time determined by your initial clearing time test, discard the fixer.
2. Use Edwal Kwik-Wet in the developer to prevent airbell holes.