=Propagation of starters.= As has been previously shown, the quality of butter depends on the kind of bacteria in the cream or in the starter added. The commercial starters contain lactic acid bacteria that have been selected with especial care; most of the starters now sold contain but a single kind of bacteria; hence, are often called pure-culture starters. The package purchased contains but a small quantity, and before the starter can be used in the ripening of cream, it must be increased in amount. It must also be propagated from day to day so that a fresh starter shall be available daily for addition to the cream. The propagation of the starter must be done with especial reference to keeping it in good condition and in as high a state of purity as possible. In the past the starter was propagated, by adding the contents of the bottle purchased to a small amount of milk that had been heated and cooled; this, if kept in a warm place, would be curdled in twenty-four hours, and could be used for the inoculation of a large mass of milk, that had been treated in a like manner, and which, when curdled, was added to the cream; a small amount was saved for the purpose of again inoculating a mass of milk that had been heated and cooled. Following this method it was very difficult to keep the culture from becoming contaminated with other forms of bacteria. More recently the most successful butter makers have propagated the so-called "mother starters" in small vessels, and have used the larger mass of starter for the inoculation of the cream alone. Glass vessels are preferable for the propagation of the mother starters since they are impervious and through the transparent wall the condition of the ripened starter can be more easily determined than in a metal or earthenware vessel. An ordinary milk bottle with an inverted tumbler for a cover, to protect the starter from contamination from the air, is a most convenient vessel.