The Constitution of Matter 75 



denser than air and at first spreads over the bottom 

 of the cylinder. Leave the apparatus untouched for 

 some time, examining the appearance of its contents at 

 intervals of 5 minutes. In spite of the considerable 

 difference in the densities of the two gases, air and 

 bromine, it will be found that the bromine vapour 

 rises slowly in the cylinder, becoming less pronounced 

 in colour as it does so on account of admixture with 

 the air. If the duration of the experiment be suffi- 

 ciently prolonged, the bromine, as shown by its colour, 

 will eventually reach the top of the vessel, and the 

 contents of the latter will be of a uniform pale brown 

 tint. 



Caution. When the experiment is finished, remove 

 the glass plate from the top of the cylinder in the open 

 air, and turn the vessel upside down at arm's length. 

 The bromine vapour will fall from the cylinder. When 

 washing the cylinder and glass tube in readiness for 

 future use, be careful to keep the nose and eyes away 

 from the sink till it is certain that no traces of bromine 

 are left. 



EXPERIMENT 25. Half fill a small glass dish with 

 concentrated hydrochloric acid solution. Add suffi- 

 cient litmus solution to a quantity of water to make 

 the liquid of a deep blue tint. Fill a test-tube with 

 this blue liquid, cover the mouth of the tube with the 

 thumb, and invert the tube in the acid. Gently 

 withdraw the thumb under the surface of the acid, 

 and fix the tube in position by means of a clamp. 

 The mouth of the tube should be about half an inch 

 below the surface of the acid. The strong acid will 

 not damage the skin if it be washed off at once. Allow 

 the apparatus to stand in a quiet place for a day or 

 two, examining the appearance of the blue liquid at 



