134 JUNIOR GRADE SCIENCE 



These proportions are remarkablj 7 constant in ordinary air, and it 

 is only in localities or under conditions of an exceptional character that 

 they vary to any noteworthy degree. In the air of mines the oxygen 

 has been found as low as 18 '6 per cent., but this represents almost the 

 lowest percentage of oxygen ever obtained from a place where human 

 beings could live. In the midst of vegetation, or open ground, especially 

 in the daytime, oxygen is present in the proportion of about 21 per 

 cent., but never more. 



Nitrogen not only serves to tone down the activity of oxygen as a 

 supporter of combustion, but it is also useful to the life of plants. A 

 few lowly plants appear to absorb nitrogen direct from the air, but the 

 majority of them obtain it indirectly as the result of the action of 

 bacteria existing in the soil or in their roots. 



The proportion of carbon dioxide rarely exceeds 3 parts in 10,000 

 in pure air, and is not often less than 2 '7 parts per 10,000. During the 

 night the proportion is slightly greater than in the day. In the streets 

 of a town the amount of carbon dioxide only exceeds the average 

 amount of the open country by about 1 part in 10,000. In rooms, 

 however, and badly^ventilated places, carbon dioxide is often greatly 

 in excess, and oxygen is present in a much smaller proportion than it 

 ought to be. Carbon dioxide is not essentially a poisonous gas, but it 

 is often found in bad company, and when it occurs in excess the air 

 of which it forms a part is unfit to breathe. 



Air always contains a certain proportion of invisible water vapour, 

 and when the air is cooled to a sufficient degree this vapour becomes 

 visible in the form of mist, fog, cloud, rain, or other familiar forms of 

 water. Ozone is a peculiar form of oxygen, and is usually present 

 in the air of the open country or over the sea, but not in that of 

 towns. 



In addition to these and other gases, numerous minute solid particles 

 are suspended in air, some of them being living germs. They are more 

 abundant in the town than in the countrv. 



61. QUANTITATIVE STUDY OF THE ACTION OF HEAT 

 ON POTASSIUM CHLORATE. 



i. Weight of oxygen given off from potassium chlorate when heated. 

 Fit up a Winchester quart bottle as shown in Fig. 97. Fill the bottle with 

 water, and blow through the short tube, so as to cause water to syphon 

 over through the tube A into the cylinder B. Fit the test-tube on the cork. 

 If the apparatus is air-tight the water will cease to rise in B. If it does 

 not, examine the corks, and detect the leak. 



Put a little manganese dioxide in the dry test-tube and a piece of asbestos 



