174 SCIENTIFIC AMUSEMENTS. 



so highly are employed for beer and pale ale. The vegetable germs 

 are introduced into the " wort " or liquor produced by treating the 

 malt with water in the form of yeast. As the action of fermentation 

 proceeds, the " yeast plant" increases in quantity, and either sinks 

 to the bottom of the vat as a sediment (when the fermentation is 

 carried out at a low temperature, producing the German " lager " 

 beers), or rises to the top mixed with froth and scum (when the 

 temperature is higher, as in the English system of brewing). 

 Large quantities of carbon dioxide gas are evolved from the 

 fermenting liquid, whilst a corresponding quantity of alcohol is 

 simultaneously formed; for bottled beer, the liquid is bottled 

 before the fermentation is quite finished, so that a further de- 

 velopment of carbon dioxide goes on in the bottle, causing the 

 beer to froth greatly on uncorking (Expt. 71). 



In the manufacture of wine, the general character of the action 

 is much the same ; for "sparkling" wines (champagne, &c.), the 

 wine is bottled, like the beer, before the fermentation is finished ; 

 whilst "still " wines (claret, sherry, port, &c.), are carefully treated 

 to prevent any such action going on after bottling. In hot 

 climates, there are usually sufficiently large quantities of yeast 

 plant germs floating about in the air to start fermentation in a 

 saccharine fluid when exposed to the air for a short time so as to 

 take them up ; thus freshly expressed grape juice will begin to 

 ferment apparently spontaneously in such a climate if kept in an 

 open vessel. If, however, the juice be pressed out into clean 

 bottles with as little exposure to air as possible, the action of 

 fermentation is greatly delayed, as the requisite quantity of germs 

 has then not been taken up ; whilst by taking extreme precau- 

 tions to avoid contact with air containing germs, the grape juice 

 may be sealed up in glass vessels, and kept for an indefinitely 

 long time without fermenting at all. 



Dissolve a quarter of a pound of moist sugar (cane sugar, con- 

 taining a little uncrystallisable sugar and colouring matter) or of 

 honey in a quart of water, and add to this solution some fresh 

 brewer's yeast, or a little German yeast rubbed up with water to 

 a cream (German yeast is the frothy yeast from brewing collected 

 on a cloth filter, drained, and pressed as dry as possible). Place 

 the whole in a flask with a cork and delivery tube attached, and 

 keep the whole in a moderately warm room. After the lapse of 

 a few hours the liquid will begin to froth, and if you then place 

 the end of the delivery tube in a wine glass containing lime water 

 (fig. 66), you will see that gas is being evolved by the fermenta- 

 tion now beginning ; and that the gas is carbon dioxide will be 

 shown by the milkiness or precipitate produced in the lime 



