Niddah, Chapter 9, Mishnah 8. The previous Mishnah taught that a doubtful stain found on a garment can be tested with seven substances known to dissolve blood: rok tafel (bland saliva), mei grisin (bean water), mei raglayim (urine), nesser (lye), boris (soap), kimonya (a cleansing clay) and ashlag (an alkaline cleansing mineral). If the mark comes out, it was blood; if it withstands all seven, it was only a dye in the fabric. Our Mishnah now defines three of those substances precisely and lays down the procedure by which the test must be performed.
Three definitions
"Eizehu rok tafel?" What qualifies as the bland saliva of the list? "Kol shelo ta'am kelum": saliva of a person who has not tasted anything. Once a person has eaten, his saliva is no longer in the state that gives it its cleansing strength; the Mishnah requires saliva from someone who has taken no food that day.
"Mei grisin": this refers to "le'isas grisin shel pol chalukei nefesh", a preparation of ground beans, split beans that have been worked and softened in water.
"Mei raglayim": not fresh urine, but "shehechmitzu", urine that has already begun to turn and sour. Only in that state does it possess the potency the test relies upon.
How the test must be carried out
"Vetzarich lechaskeis shalosh pe'amim lechol echad ve'echad": every one of the seven materials must be worked vigorously into the mark on three separate occasions. One pass with a given substance does not suffice. Each is granted its full chance to dissolve whatever is in the fabric before the next is brought to bear.
Sequence matters no less. Suppose he ran the substances over the mark in some arrangement other than the one the list prescribes ("he'eviran shelo kesidran"), or suppose he took all seven and used them simultaneously rather than one following the next ("o shehe'evir shiv'ah samanim ke'echas"). The ruling in either case is "lo asah velo kelum": nothing whatsoever has been achieved. The examination counts for nothing, and the stain stands in doubt just as it did before he began.
The Mishnah is teaching that a halachic test is only as good as its faithfulness to its own rules. The strength of the seven substances lies not only in what they are, but in the disciplined way they are used: each one properly prepared, each one applied three times, and each one in its turn.