144 SCIENTIFIC AMUSEMENTS. 



Production of Colours by Chemical Changes due to Solution 



of Gas. 



As a general rule, no marked alteration in colour attends the 

 absorption of a gas by a fluid, as long as no precipitation is brought 

 about ; but some exceptions occur. Thus, in Expt. 114, the action 

 on copper of ammonia gave rise to solution of copper, and forma- 

 tion of a blue liquid when air was also allowed access, but not in 

 the absence of air. 



Although we are accustomed to regard the atmosphere as a 

 typical example of a gaseous substance, yet really more than one 

 gas is present, in even the purest open-country air ; whilst the more 

 or less vitiated air of a town, or close unventilated room, really 

 contains a number of gases. In all cases, however, two gases 

 greatly predominate, called oxygen and nitrogen respectively. The 

 latter roughly constitutes about |- of the air and the former \ ; but 

 although oxygen is not present in the larger quantity, it is by far 

 the more important of the two, as it is the oxygen and not the 

 nitrogen which is necessary to keep up animal life by breathing. 



Expt. 148. To procure a dark-coloured Fluid by Absorption 

 of Oxygen from the Air. In Expt. 114 what happens is that 

 the oxygen present in the air dissolves in the liquid and exerts a 

 chemical action on the copper, converting it into a compound 

 (oxide of copper), which dissolves in the ammonia- water, forming a 

 blue fluid ; so that ultimately the oxygen becomes removed from 

 the air to a greater or lesser extent, thus affording the means of 

 preparing nitrogen gas (vide Expt. 184). A precisely analogous 

 action takes place if a little pyrogallic acid (as much as will lie on 

 a shilling) be put into an eight-ounce wide-mouthed bottle, and 

 then a tablespoonful of caustic soda solution poured in ; quickly 

 insert a well-fitting cork (previously greased to make it air-tight) 

 and shake up. The pyrogallic acid dissolves in the caustic soda 

 solution, forming a fluid which immediately absorbs oxygen if in 

 contact therewith, but not nitrogen, forming a dark reddish-brown 

 fluid in so doing. After a few minutes shaking the whole of the 

 oxygen present in the air originally contained in the bottle will 

 be dissolved and chemically acted upon; if now the bottle be 

 turned mouth downwards in a basin of water, and the cork 

 cautiously removed, still under water, as in the last experiment 

 (147), it will be noticed that the water will to some extent rush 

 into the bottle, being driven in by atmospheric pressure, somewhat 

 as in Expt. 77, but not so violently ; on comparing the quantity of 

 water thus driven in (again corking the bottle under water before 

 removing it for inspection) with the quantity of air originally 



