THE RECOGNITION OF SUBSTANCES, ETC. 381 



Examination of a Gas. The gas has a color. A brown 



gas may be bromine or nitrogen tetroxide. The former liberates 

 iodine from potassium iodide solution (p. 205), but not bromine 

 from a bromide (insert rods moistened with solution of an iodide 

 and a bromide). The latter becomes deeper brown on warming 

 (p. 311). A greenish-yellow gas is chlorine. It bleaches, and 

 displaces bromine from a solution of a bromide (p. 202). 



The gas may become colored (yellow or brown) on admitting 

 air. It is nitric oxide (p. 311). 



The gas may have a distinctive odor. Sulphur dioxide, hy- 

 drogen sulphide, nitrogen tetroxide, and ammonia are of this 

 kind. 



The gas may fume in the breath. The chloride, bromide, and 

 iodide of hydrogen do so. Distinguish by dissolving in little 

 water and adding manganese dioxide. 



The gas may be combustible. Burning with a blue flame in- 

 dicates hydrogen (vessel bedewed with moisture), or carbon mon- 

 oxide (leaving carbon dioxide, test, p. 336). Burning with a 

 slightly luminous flame indicates methane (p. 345) and a very 

 luminous flame (often depositing carbon) indicates ethylene (p. 

 350) or acetylene (p. 351). 



The gas may relight a glowing splinter of wood. This is oxy- 

 gen, or nitrous oxide (p. 315). The former, with nitric oxide, 

 gives a brown gas (p. 311), the latter does not. 



The gas may give a white precipitate (CaCO 3 ) with lime- 

 water. This is carbon dioxide. 



The gas having none of these properties is nitrogen (p. 287). 



In most cases other distinctive properties will be found on the 

 pages referred to. 



Exercises. 1. Look up the references, and give the proper- 

 ties (physical as well as chemical), other than those mentioned in 

 p. 377, by which you should recognize: (a) sulphur, (b) carbon, 

 and (c) iodine. 



