There's nothing I hate more than lore. Lore is stuff that's well-known, but untraceable. Examples of photographic lore are "simple table salt can be used to clear fixer," or "John Herschel was the first to notice the fixating action of thiosulfates upon silver salts." But from every idiot with a blog on the internet, to the greatest minds of photographic writing, very few people ever bother to cite properly, and merely parrot. The problem is often that they never read the primary material to begin with. Well, I did my homework, and I finally traced the most important paper in the birth of photography: the discovery of fixer by John Herschel in his paper "On the Hyposulphurous Acid and its Compounds." The Edinburgh Philosophical Journal, Vol. 1 (8 Jan. 1819): 8-29. (available from archive.org) The paper begins thus (my emphasis): This bitterness is the triggering factor: not being able to account for the sudden appearance of bitterness in a compound he already knew, Herschel sought to figure out what chemical transformations had happened in his solution, and how he could characterize it. After a quick trip to the library, our man found some clues as to the possible composition of his solution. His compound could not be "sulphuric" or "sulphurous" (the two forms of sulfur compounds he knew), and he found the excluded third in an experiment on the decomposition by atmospheric exposure of his original solution. "Hyposulphurous Acid" had been created in the solution, and while this acid is not available alone, it can readily be found attached to a base in the form of "hyposulphites". However, the literature already available was rather bad, vague, or incomplete with respect to the properties of these compounds. Thus began the quest to study the properties of "hyposulphites". In the paper, Herschel decribes their taste ("intensely sweet or intensely bitter"), what precipitates they form, etc. But must importantly, one notable property of all "hyposulphites" is their ability to dissolve "muriate of silver" or silver salts (not sure if he means strictly AgCl or all AgX, here). On page 19, the first clear mention of this property is thus given: I merely glanced over this part the first time I read the paper because I had no idea what "muriate" was. Although Herschel, in another paper, kept referring to this article as his discovery of hypo, I couldn't connect it with the birth of photography, and was looking for other papers, until an internet search finally helped me connect "muriate" with "halide" or "salts". Later in the paper, Herschel describes thus the impact of dissolved silver salts on hypo: There you have it: Herschel's test to know if your fixer is fresh or exhausted. Bitter = fresh ; sweet = exhausted. Easy! The 1819 paper makes no mention of photography, but it is well known that afterwards the word got out, and people like Talbot and Daguerre added 2+2: silver salts, reducing to silver after exposure to light, soluble in hypo = fixed photos! Thanks to printing, we have a record of the development of knowledge, and we can also witness that earlier scientists still used their bodies as fundamental measuring instruments. If you're into the study of smell, the Luca Turin saga on the vibrational nose also begins with the quest to figure out what two different chemical compounds smell like, even the toxic, dangerous ones (cf. The Emperor of Scent for a general public version and The Secret of Scent for the heavier version). In photography, we are indebted to a man's taste buds and their ability to transform all sorts of chemical information into subjective impressions, triggering a revolution with a lasting legacy.