14 STUDIES IN PHYSIOLOGY 



of the water ; at length they disappear altogether. Mean- 

 while the water has been slowly rising into the bottle. 

 When this upward movement has ceased, we cover with a 

 glass plate the mouth of the bottle and quickly turn it right 

 side up, thus keeping within the bottle the water that arose 

 from the tray. 



A Test for Acids. 1 Let us first examine the water we have 

 collected in the bottle. If we drop into ordinary water a 

 bit of paper treated with a kind of vegetable stain called 

 blue litmus, the color remains unchanged ; but if we dip the 

 blue paper into any kind of acid substance, as lemon juice, 

 or vinegar, the color becomes red. The presence of acids, 

 then, may be readily demonstrated by using blue litmus. 

 When we test in this way the water we obtained in the pre- 

 ceding experiment, we find it is acid. The white fumes of 

 oxid of phosphorus, which we saw settling upon the surface 

 of the water, have been dissolved and have made the water 

 acid in its properties. 



The Composition of Air. While the phosphorus was burn- 

 ing, it continually took away oxygen from the air confined 

 in the bottle until all the oxygen was used. Hence, after 

 the oxid of phosphorus thus formed had been dissolved in 

 water, a space was left. But the pressure of the atmosphere 

 on the surface of the water outside the jar caused the water 

 to rise and fill this space as fast as the oxygen was with- 

 drawn. The water, therefore, comes to occupy the space in 

 the bottle which, at the beginning of the experiment was 

 taken up by oxygen. If we measure the capacity of the 

 bottle and then measure the amount* of water, we see that 

 the latter takes up about one fifth of the space in the bottle. 

 This means that air is composed of about one fifth oxygen 

 and four fifths nitrogen (see Fig. 3). (Much more accurate 

 results can be obtained if a piece of phosphorus is floated 

 on the cork and allowed to oxidize slowly within the bottle 

 of air for two or three days.) 



1 See "Laboratory Exercises," No. 7. 



