220 VINEGAR, CIDER, AND FRUIT-WINES. 



marsh gas (CH 4 ), olefiant gas (C 2 H 4 ), tetrylene (C 4 H 8 ), and 

 volatile oils, such as benzol (C 6 H 6 ), toluol (C 7 H 8 ), naphthalin 

 (C 10 H 8 ), paraffin (C 20 H 42 ), phenol (C 6 H 6 O), etc., are given off. 

 The actual facts which are observed in the distillation of wood 

 are as follows : 1. The water passes off which is extraneous to 

 the wood ; 2. The wood itself is decomposed and gives rise to 

 water and the crude acetic acid, which is next eliminated; 3. 

 Condensable matters containing an excess of carbon forming the 

 tar and oily substance pass over ; 4. Toward the close of the 

 operation, carbonic oxide and marsh gas are evolved, leaving in 

 the retort a charcoal similar in form to the wood introduced. 



Distillation of Wood. 



The distillation of wood is carried on in retorts made of cast- 

 iron or wrought-iron, and sometimes of clay. The latter have 

 the advantage of not burning through, but it is difficult to keep 

 them entirely tight, a number of small cracks being formed 

 through which a portion of the vapors escapes. Cast-iron re- 

 torts do not readily burn through, and are but little affected by 

 the vapors of the acid, but they frequently burst, and defective 

 places are difficult to repair. Wrought-iron retorts gradually 

 burn through on the bottom, where they come in contact with the 

 fire ; they can, however, be repaired by riveting strong boiler- 

 plate upon the defective place, and are less affected by the acetic 

 acid vapors than might be supposed, because some protection is 

 afforded to them by the deposition of a layer of coal from de- 

 composed tar vapors and gases upon their interior surface. Ex- 

 perience having shown that wrought-iron is more strongly 

 attacked on the less hot places than on the hottest, it is customary 

 to provide wrought-iron retorts with cast-iron doors and discharge 

 apertures. To retard burning through, the exterior of the re- 

 torts is coated with a thick layer of lime or clay, or with a mix- 

 ture of lime, iron-filings, and water glass. The riveting must 

 be carefully executed, and the joints luted with clay or trass- 

 mortar. 



Form of the retorts. Clay-retorts are mostly Q -shaped like 

 those for the manufacture of coal-gas. 



