42 EXPERIMENTAL PLANT PHYSIOLOGY 



the jar, therefore some of the gas must have been dis- 

 solved in the water. 



3. If a Uttle moist blue litmus-paper is put into the 

 gas, or, better still, into the solution of the gas in water, 

 the litmus-paper slowly turns red. 



4. If a lighted taper is put into the gas it is immedi- 

 ately extinguished. 



To show well the extinguishing power of carbon- 

 dioxide ignite some turpentine in a saucer, then invert 

 a bell-jar filled with the gas over the flames. 



5. The gas can be collected by downward displace- 

 ment of air. It is therefore heavier than air. 



6. If a little lime-water is poured into one of the jars, 

 or if the gas be allowed to bubble from the end of the 

 delivery-tube into some lime-water, the lime-water 

 becomes milky. 



Inferences. — Carbon-dioxide is an invisible, faintly 

 smelling gas. Its solubility in water is greater than 

 that of oxygen. It is slightly acid. It will neither 

 burn itself nor allow other substances to burn in it. It 

 is heavier than air and turns lime-water milky. 



Summary. — The knowledge that has now been gained 

 respecting the composition of the air may be summar- 

 ised as follows : 



It is a mixture of gases. 



About four-fifths of the whole is the inert gas 

 nitrogen. 



About one-fifth is oxygen, a gas in which substances 

 very readily burn. 



Carbon-dioxide, a heavy gas in which substances will 

 not bum, is present in varying but small amount. 



(The quantity differs according to the locality. In 

 country air there are from 3 to 4 parts in 10,000, but 

 in towns the proportion is greater.) 



In addition water-vapour is present. 



The air also contains other rare gases and various 

 impurities, but for the purposes of this book these need 

 not be taken into account as they are present in such 

 small quantities. 



