40 INTRODUCTION TO GENERAL SCIENCE 



tubing with another glass tube placed in a test tube, and place 

 the test tube in the beaker half full of cold water. Boil the 

 water in the flask only so fast that no steam will come from 

 the top of the open test tube. Compare the condensed water 

 with the water which is in the flask. Taste a little and state 

 how it seems. Place your thumb over the top of the test 

 tube and shake the distilled water for a short time. Taste 

 again and explain the results. 



b. Mix a tablespoonful of molasses with a pint of water and 

 half a yeast cake, and allow fermentation to take place for two 

 days. Use the resulting mixture in the place of the dirty salt 

 water, but heat more gently. As soon as you obtain a few 

 drops of distilled liquid, pour it out on the flat base of a ring 

 stand and touch a match to it. What happens ? What is it ? 



28. DESTRUCTIVE DISTILLATION 



In the preceding parargaph we considered simple distilla- 

 tion, in which the material was not changed but merely sepa- 

 rated physically. It is possible, however, to carry distillation 

 so far that the substance heated is broken up chemically, and 

 the resulting products may be entirely different from the 

 original substance. This is called destructive distillation, 

 and the products obtained by this process are often very 

 valuable. 



The destructive distillation of soft coal gives gas, coal tar, 

 coke, carbon, and other by-products. Wood yields a gas, 

 wood tar, acetic acid, and wood alcohol. Petroleum yields 

 several volatile liquids, such as gasoline, naphtha, benzine, 

 and kerosene. The solid remainder is paraffin in some petro- 

 leums and asphalt in others. See Section 137, Coal, Soft and 

 Hard, and Section 138, Petroleum and Natural Gas. 



