12 STUDIES IN PHYSIOLOGY 



the glass plate from the month of the bottle when it is just 

 over the end of the delivery tube. The bottle still remains 

 filled with water. We have learned (p. 6) that the atmos- 

 phere above the earth is about fifty miles in thickness. On 

 every square inch of the earth's surface at the level of the sea 

 this air presses down with a weight equal to about fifteen pounds. 

 Hence if our tray happens to measure seven by ten inches 

 (seventy square inches), the atmospheric pressure on the 

 surface of the water will be seventy times fifteen pounds, or 

 ten hundred and fifty pounds, which is over half a ton. It is 

 this downward pressure of the air on the surface of the water 

 that keeps the bottle filled, since the glass bottom of the bottle 

 prevents a like downward pressure on the water within. 



Collection of Oxygen. If now we continue to heat the 

 mixture of chlorate of potassium and oxid of manganese, 

 the bubbles of oxygen will rise into the bottle and gradually 

 displace the water. When the bottle is filled with the gas, 

 we close it with the glass plate, and turn it right side up. 

 After we have filled several bottles in the same way, we are 

 ready to determine some of the characteristics of this most 

 important element. 



Physical Properties of Oxygen. % physical properties 

 we mean those characteristics of a substance that can be 

 determined by the senses. If the experiment described 

 above has been carried on slowly, and if the gas is passed 

 through bottles of water and caustic soda, the oxygen is 

 seen to be colorless; when we lift the cover and smell of the 

 gas, it is found to be odorless ; and by inhaling some into 

 our mouth we learn that it is tasteless. We might of course 

 expect these results, since fresh air, in which oxygen is 

 present, is likewise colorless, tasteless, and odorless. 



Chemical Properties of Oxygen. The striking chemical 

 property of oxygen is shown by the following experiments. 

 Let us set fire to a piece of wood and then extinguish the 

 flame, leaving a glowing spark at the end. Carefully 

 removing the glass plate from one of the bottles of oxygen, 



