120 



FIRST YEAR SCIENCE 



Fig. 58. 



E 



tube to slip or admit air. Completely close the rubber tube with a 

 Hoffman's screw just beyond the place where it leaves the glass tube. 

 Placing this closed end in a large dish so as not to waste any 

 mercury, fill the glass tube with mercury. Place the thumb 

 over the open end of the tube and invert it in a cup of mer- 

 cury. If the connections were made tight, the mercury will 

 not fall far below the end of the glass tube. The air pres- 

 sure keeps the mercury up. This is a simple form of 

 barometer. 



While the tube is still standing in the mercury cup take 

 another glass tube similar to the first and attach it to the 

 open end of the rubber tube in the same way as the first was 

 attached. Place the free end of this tube in a dish of colored 

 water and gradually open the Hoffman's screw. The water 

 rises in the tube. Why? What is meant by sucking water 

 up a tube ? 



Experiment 68. Fill a bottle with clean water and fit it 

 tightly with a rubber stopper having two holes in it. Plug 

 one of the holes tightly with a glass tube one 

 end of which has been closed by heating in a 

 Bunsen burner. Through the other hole put 

 an open glass tube 10 to 15 cm. long. See 

 that both tubes fit tightly in the stopper and 

 that the stopper fits tightly 'in the bottle. 

 Now attempt to " suck " the water out of the 

 bottle through the open tube. Does it come 

 out freely? Pull out the glass plug. Does 

 it come out any better? If so, why? 



The mercurial barometer we have already made 

 in a rough form. The best form of these instru- 

 ments consists of a glass tube of uniform bore 

 about eighty centimeters long and closed at one 

 end. After being carefully filled with pure mer- 

 cury, it is inverted in a cistern of mercury. The 

 cistern of mercury has a sliding bottom easily 

 moved up and down by means of a set screw. At 

 the top of the cistern there is a short ivory peg. Flg: 59< 

 The lower end of the ivory peg is at an exactly measured 



