140 GENEEAL SCIENCE 



which is produced by the use of sodium bicarbonate (baking soda) 

 and sulphuric acid. Another type of this kind of fire extinguisher 

 contains material which forms bubbles or foam. Oil fires are thus 

 extinguished easily as the bubbles prevent air reaching the surface of 

 the oil. 



When the fire extinguisher is inverted the stopper falls out of the bottle 

 and the sulphuric acid and baking soda come together, forming carbon dioxide 

 and water, which are forced out of the rubber hose on the fire. Fire extinguish- 

 ers should be tested from time to time to see that the acid has not deteriorated. 



Experiment. The action of carbon dioxide on a flame may be seen easily by 

 placing in an olive bottle a few spoonfuls of baking soda, a little water and a 

 small amount of sulphuric acid. Close the bottle with a one-hole rubber stop- 

 per through which runs a glass tube extending into an empty bottle. The 

 bottle will soon be full of the gas generated from the first bottle. Light several 

 candles and pour the gas over them. Try a bottle full of the gas on a Bunsen 

 burner. 



Kindling Temperature and Matches. Substances which burn 

 have a definite temperature at which they will start burning. Not 

 all substances start to burn at the same temperature. Wood, 

 phosphorus, paper, sulphur, etc., have different kindling points. 

 The match is an example of the use of different substances with 

 different kindling points. 



Matches are made by soaking one end of a piece of wood in 

 paraffin and then dipping this in a mixture of glue, phosphorus, and 

 a material which gives off oxygen, such as potassium chlorate. Some 

 matches are made of powdered glass, zinc oxide, 

 rosin, glue and coloring matter. Safety matches 

 have the phosphorus on the box so that the matches 

 will not take fire inside the box. 



Experiment. To Boil Water in a Paper Bag. Take a square 

 piece of paper and fold it so as to form a conical bag as 

 shown in Fig. 109. Suspend the bag by strings and, pouring 

 water into it, allow the flame of an alcohol lamp or Bunsen 

 burner to fall on the bag, being careful to prevent the flame 

 from touching the paper in any place where there is no water. 

 The water can now be heated until it boils, without the paper 

 FIG. 109. being burned, because the paper cannot be heated much 

 more than 212 F., and this is not sufficient to burn it, since 

 the kindling point of paper is higher than the temperature of boiling water. 



