Harley Procter totaled the ingredients which did not fall into the category of pure soap — they equaled 56/100%. He subtracted from 100, and wrote the slogan "99-44/100% Pure®: It Floats." ...

A bar of soap, invented almost 150 years ago and still selling in stores today, has become a part of Americana largely by advertising its two strange merits: It floats and its “99+44⁄100% Pure.” ...

Look for Fels Naptha Laundry Soap bars or something similar. That said, the Gore-Tex people themselves say it’s OK to use either a powder or liquid, and most other gear makers go along with that.