of its Brewers, for that its ill Taste is lost in nine or twelve Months, by the Age of the Beer, and the strength of the great Quantity of Hops that were used in its preservation. The hydrometer transformed how beer was brewed. Before its introduction beers were brewed from a single malt: brown beers from brown malt, amber beers from amber malt, pale beers from pale malt. Using the hydrometer, brewers could calculate the yield from different malts. They observed that pale malt, though more expensive, yielded far more fermentable material than cheaper malts. For example, brown malt (used for Porter) gave 54 pounds of extract per quarter, whilst pale malt gave 80 pounds. Once this was known, brewers switched to using mostly pale malt for all beers supplemented with a small quantity of highly coloured malt to achieve the correct colour for darker beers. The invention of the drum roaster in 1817 by Daniel Wheeler allowed for the creation of very dark, roasted malts, contributing to the flavour of porters and stouts. Its development was prompted by a British law of 1816 forbidding the use of any ingredients other than malt and hops. Porter brewers, employing a predominantly pale malt grist, urgently needed a legal colourant. Wheeler's patent malt was the solution. Louis Pasteur's 1857 discovery of yeast's role in fermentation led to brewers developing methods to prevent the souring of beer by undesirable microorganisms. In 1912, the use of brown bottles began to be used by Joseph Schlitz Brewing Company of Milwaukee, Wisconsin in the United States. This innovation has since been accepted worldwide and prevents harmful rays from destroying the quality and stability of beer. Modern beer Many European nations have unbroken brewing traditions dating back to the earliest historical records. Beer is an especially important drink in countries such as Belgium, Germany, Austria, Ireland, the UK (England, Wales, Scotland and Northern Ireland), France, the Scandinavian countries, Poland, the Czech Republic, Spain and others having strong and unique brewing traditions with their own history, characteristic brewing methods, and styles of beer. Unlike in many parts of the world, there is a significant market in Europe (the UK in particular) for beer containing live yeast. These unfiltered, unpasteurised brews are more challenging to handle than the commonly sold "dead" beers; "live" beer quality can suffer with poor care, but many people prefer its taste. While beer is