318 CHEMISTRY. 



435. Dry Distillation of "Wood. These products can be 

 obtained separate from each other by a process called dry 



distillation, repre- 

 sented in Fig. 109. 

 Some pieces of 

 wood are heated 

 in a retort, and the 

 volatile matters 

 pass over through 

 the tube into a 

 receiver. The il- 

 luminating gas passes on through the bent tube, and is col- 

 lected in the usual manner. One of these, the wood-tar, is 

 very thick ; and the other, the wood-vinegar, is a thin, wa- 

 tery substance. Charcoal, not being at all volatile, is left 

 behind in the retort. Wood is composed of the three ele- 

 ments, carbon, oxygen, and hydrogen ; and it is out of these 

 that the products above mentioned are formed by the in- 

 complete combustion produced by the heat. They did not 

 exist in the wood, and therefore may be properly called 

 products. Even the carbon left in the retort may be called 

 a product, for in the wood it does not exist as carbon, but 

 is in combination with the other elements forming the com- 

 pound, wood; just as oxygen does not exist in water as 

 oxygen, but is in combination with hydrogen, forming the 

 compound, water. 



Two of the above products charcoal and illuminating 

 gas have already been sufficiently described in other parts 

 of this book, and therefore we will now notice only the other 

 two. 



436. Pyroligneous Acid. The name of this acid is de- 

 rived from a Greek word, pur, fire, and a Latin word, lig- 

 num, wood. Its acidity comes from acetic acid, and hence 

 the propriety of calling this liquid wood-vinegar, Its pe- 



