178 SCIENTIFIC AMUSEMENTS. 



ligneous matter of certain kinds of sawdust renders the chemical 

 action less rapid. Alcohol has thus been made commercially from 

 sawdust, but of somewhat inferior quality ; that from pine saw- 

 dust is said to have a natural resinous flavour, so as to form a sort 

 of gin. As with starch, the converting action of the sulphuric acid 

 is a good deal quickened by heating the materials in pressure 

 boilers rather than in open vessels. 



In the ordinary process of baking household bread, the flour is 

 mixed with water and some yeast to a thick paste or " dough," which 

 is then set by in a somewhat warm place until the dough begins to 

 " rise." After the " working " has gone on for some time the dough 

 has greatly increased in bulk, and has become honeycombed all 

 over with bubbles. This arises from the action of the yeast upon 

 the starchy matter present, first converting it into sugar, and then 

 splitting up the sugar into alcohol and carbon dioxide ; this latter 

 swells up the dough and makes it porous, so that when baked, bread 

 of the usual vesicular structure is produced, instead of a solid mass 

 like a suet dumpling. One result of this is that all bread prepared 

 with "leaven" is to some extent impregnated with alcohol, so that 

 by taking a portion of the crumb from the interior of a newly baked 

 loaf, soaking it in water, and distilling the fluid, minute quantities 

 of alcohol can be extracted, sufficient to recognise by chemical tests. 

 Most of the alcohol formed during the rising of the dough, how- 

 ever, is driven off by the heat of the oven, so that only very small 

 quantities remain in the baked bread. 



In the preparation of bread on the large scale the baker's men 

 sometimes knead the dough by treading ; in any case a good deal 

 of handling is required. As this is by no means an appetising 

 operation, machines have been constructed to take the place of 

 this mixing and kneading by hand or foot. What is termed 

 " aerated " bread is prepared in a different way. Flour is mixed 

 in a specially constructed machine with water impregnated with 

 carbon dioxide under considerable pressure, this pressure being 

 maintained during the mixing; when the dough is thoroughly 

 mixed the pressure is relaxed, and consequently a good deal of gas 

 is liberated, just as when a bottle of soda water is opened ; this 

 swells up the dough and makes it porous, just as though leaven 

 had been used. Aerated bread, consequently, contains no alcohol, 

 and from the absence of leaven in it is often considered more whole- 

 some for persons suffering from weak digestion and other ailments, 

 besides being made in a far more cleanly way than ordinary leavened 

 bread. Owing to the different mode of preparation, it often has a 

 slightly different taste ; this is partly due to the fact that the action 

 of the yeast in the ordinary process of bread making converts a 



