246 Domestic Science 



The nature of the gas which has been collected may 

 be tested after allowing the water and gas in the measur- 

 ing tube to cool. We already know that caustic potash 

 solution readily absorbs carbon dioxide. After reading 

 the volume of gas collected in the tube, with the water 

 inside and outside the tube at the same level, insert a 

 small stick of caustic potash under the mouth of the 

 tube, close the mouth with the thumb and remove the 

 tube from the water. Shake the tube till all the potash 

 has been dissolved and then remove the thumb under 

 water. Read off the volume of gas remaining. 

 The difference between the two volumes will be the 

 amount of carbon dioxide contained in the gas collected 

 from the water. Part of this carbon dioxide was 

 dissolved in the water as such, and part in the form of 

 compounds which are decomposed by heating them to 

 the temperature of boiling water. (See Part II, Hard- 

 ness of Water.) 



The remainder of the gas in the tube consists almost 

 entirely of oxygen and nitrogen. To determine roughly 

 their relative amounts, put about O'l g. of pyrogallic 

 acid the " pyro " of the photographerinto a filter- 

 paper, roll it up tightly, and insert it under the mouth 

 of the tube. Close the tube as before with the thumb 

 and remove it from the water. The pyrogallic acid 

 will dissolve readily and, in the presence of the 

 caustic potash already in solution, will quickly 

 absorb the oxygen, if the tube and its contents be 

 gentry shaken. The liquid will assume a deep brown 

 tint as 'a result of this absorption. After shaking for 

 about five minutes remove the thumb under water and 

 again read off the volume of gas which remains in the 

 tube. This may be taken as representing the nitrogen 

 in the gas originally collected, while the volume which 



