34-- 

 Carbonic Acid. 



Most of the children have learned that there is carbonic acid in the 

 air; that it conies from the lungs in air breathed out, and that it is made 

 by burning lamps and candles. 



As a good test for the gas will be of great service, it vs^ould be better 

 at the very start to explain how lime water is so used. Lime water is 

 readily made by putting some lime into water and allowing it to stand 

 until the excess settles, leaving the solution above, clear. Pour off the 

 clear liquid for use. It can be purchased ready made at the drug store. 



A small amount of carbonic acid shaken up with the lime water makes 

 a white substance in the water, thus giving a milky appearance. The 

 substance formed is carbonate of lime. 



One of the children can breathe air through a small amount of lime 

 water in a test tube or other glass vessel. The lime water turns white, 

 proving the presence of carbonic acid. 



A bit of candle is placed in the bottom of a glass tumbler, which is 

 covered with a book. The candle soon goes out. Test the gas left with 

 lime water. 



An inverted glass is held over the chimney of a burning lamp. Slip 

 a card over the mouth and test the carbonic acid. The gas coming 

 from various burning substances may be tested. 



To make carbonic acid in a quantity unmixed with air, etc., pound up 

 into small pieces limestone or marble; place in a wide-mouthed bottle or 

 flask, into which is fitted a cork and delivery tube. Cover the marble 

 with water, then pour in some muriatic or sulphuric acid. Bubbles of 

 carbonic acid come off rapidly. As it is heavier than air of the same 

 temperature, it can be caught in empty jars or bottles standing upright, 

 lightly covered with cards. When a few jars have been obtained, try 

 them with lime water, lighted candles, lighted splinters, etc. Show that 

 the gas is heavier than air by pouring carbonic acid into a jar which 

 contains only air, then testing this jar's contents; or pour some into ajar 

 at the bottom of which is a lighted candle. 



Many other experiments can be made with the carbonic acid, making 

 the pupils familiar with its properties. 



Diffusion of Gases. 



That gases diffuse themselves out into the air from the vessels which 

 contain them, may be shown by leaving a jar of carbonic acid uncov- 

 ered for a short time. Although heavier than air, it leaves the jar. An 

 inverted jar of oxygen will show the same. The lighter oxygen will not 

 stay in the jar. The same is true with illuminating gas, vapor of gaso- 

 line, of ether, etc. These experiments make it clear why in the air the 

 different gases are thoroughly mingled instead of the heavier settling to 

 the bottom and the lighter going to the top. 



