Yeast 1850 - 1880

Walker & Homfrays Embee Amber Ale label.

It was in this period that Pasteur unlocked the mystery of fermentation and the true nature of yeast as a living microorganism. There had been much debate about the process of fermentation and the role played by yeast for decades. Brewers did, however, know how to work with yeast, despite not properly understanding exactly what it was.

Some still thought yeast was a type of catalyst and its action purely chemical. This was the theory propagated by the German Leibig. 

"The chief constituents of yeast are vegetable gluten, and a small quantity of albuminous matter. When this ferment is introduced into the wort, it stimulates similar principles therein, and these exciting causes act and re-act upon each until one or both are destroyed."
Source: "The Brewer" by William Loftus, 1856, page 43.

Whatever yeast was, the changes it provoked in a sugary liquid were understood: 

"oxygen, two parts of which unite with four of carbon, and six of hydrogen in forming alcohol; while four parts of oxygen unite with two of carbon, and form carbonic acid gas. These two substances did not previously exist, but are new products of the decomposition od sugar."
Source: "The Brewer" by William Loftus, 1856, page 43.

Though they might not have known its true nature, brewers understood that using the same yeast was important to maintain the characteristic flavour of their beer. They maintained a supply of yeast by harvesting it from fermenting wort. (Source: "The Art of Brewing" by Frank Faulkner, 1876, page 114.) Pitching yeast was referred to as "store". 

"The best yeast for store purposes is that derived from a beer when nearing its final attenuation point, yeast which has not been exposed to atmospheric influences for any length of time, through forming the upper surface of the floating head, and as a rule the best pitching store for distinctive beers is that taken from brews of like quality, or from a beer of medium gravity."
Source: "The Art of Brewing" by Frank Faulkner, 1876, page 114. 

It's worth remembering that "medium gravity" at the time would have meant a beer of 1060-1070º.

Yeast was preserved by running cold water over it to remove any sugars and then drying and pressing it. (Source: "The Art of Brewing" by Frank Faulkner, 1876, pages 115-116.)