CHAPTER XX 

 INCIDENTAL PRODUCTS 



A BY-PRODUCT is some substance or article produced in the 

 course of a process of manufacture, in addition to the prin- 

 cipal product or material. It frequently happens, however, 

 that the by-products become of more importance than the 

 primary substances themselves. There is hardly a line of 

 manufacturing of any importance which does not yield by- 

 products; the elimination of waste is one of the great prob- 

 lems of the day. 



402. Some examples of by-products. One of the most 

 interesting fields for study of by-products is that of illumi- 

 nating gas. Tin's is ordinarily obtained by heating coal in a 

 closed retort. The heating process drives off a large number 

 of substances, leaving behind the solid coke, an article that 

 has become exceedingly important in many fundamental in- 

 dustries. Later, in the process of purification of the illumi- 

 nating gas, other by-products are obtained, such as ammo- 

 nium sulfate, which is used as a fertilizer; acetic acid, the 

 substance which makes vinegar sour; benzene and carbolic 

 acid, familiar to all; the cresols, used as disinfectants; picric 

 acid, one of the most important materials in the manu- 

 facture of explosives; aniline, the source of the best dyes; 

 and asphalt, used in paving roads and streets. The list is 

 given to indicate the importance of utilizing what once were 

 considered waste products, having no value whatever. 



403. By-products in horticulture. The record of achieve- 

 ment already indicated in the illuminating gas industry can 

 be repeated in many other fields. In horticulture itself there 



290 



