38 



CHEMISTRY FOR AGRICULTURAL STUDENTS 



To purify the gas thus obtained, pass it through the U tube, 

 B, filled with pieces of pumice soaked with solution of potash 

 (i gram of caustic potash dissolved in 3 c.c. of water) ; and to 

 dry it, through the U tube, C, filled with lumps of calcium 

 chloride. When all the air has been 

 expelled from the apparatus, and 

 pure hydrogen issues from D, ignite 

 the gas, and, by means of an as- 

 pirator, draw the products of com- 

 bustion through a small thistle 

 funnel into a U tube half immersed 

 in cold water (Fig. 22). Is the 

 liquid, which condenses in the U 

 tube, water? Has it the physical properties of water? To 

 answer this question, run it into a small flask, fitted with a ther- 

 mometer, and having an aperture to allow steam to escape, 

 and determine its boiling point. If this be 100° C, it is water. 

 Water must therefore be the oxide of hydrogen. 



Hydrogen and air form a dangerously explosive mixture. To ensure 

 absence of air from the gas before lighting it at the jet, collect a test-tube 

 by upward displacement of air, and ignite it. If air is still present, an 

 explosion will be produced ; if the gas is pure, it will burn quietly. 



Fig. 22. 



X. HYDROGEN GAS 



It has been found that hydrogen differs from oxygen, nitrogen, 

 and carbonic acid gas, in being combustible. It is also 

 lighter in weight, for it has been possible to collect it by 

 upward displacement of air. Compare its density with that 

 of air. The density of gases may be compared, like liquids, 

 by weighing equal volumes. Attach the flask (Fig. i) to the 



