SCIENTIFIC READINGS. 119 



179. Will vegetation proceed in an exhausted receiver? 



180. What effect is produced by the condensation of 

 air ? 



181. Describe the Condensing Syringe. 



182. On what principle does the Air-gun depend ? 



183. Why does the quicksilver fall in a Barometer 

 when it is about to rain ? 



184. Describe the Barometer, and explain the prin- 

 ciple. 



185. For what other purpose is a Barometer nsed be- 

 sides that of ascertaining the probable change of the 

 weather ? 



*185. Required the density of the air at twenty-eight 

 miles above the earth's surface. 



186. Required the pressure of the atmosphere on six 

 square inches of surface. 



187. Required the weight of the atmosphere in tons. 



188. If the atmosphere of Mars be twice as dense as 

 that of the earth and reckoned as high, what will be 

 its weight, supposing the diameter of Mars to be 400O 

 miles ? 



189. What difference would there be in the height of 

 the quicksilver in a barometer at the top of the highest 

 mountain in the world, to what it would be at the bot- 

 tom of the mountain ? 



WATER. 



190. In what different states does water exist ? 



191. What is Steam or Vapour ? 



192. What is the simplest state of water ? 



193. Is ice heavier or lighter than water ? 



194. What are the component parts of water ? 



195. What \tas the opinion of the ancients respecting: 

 water ? 



196. Give proofs of the universality of water. 

 197- Which is the purest kind of water? 



198. What are the opinions of philosophers relative to 

 the sea's acquiring its saltness ? 



