270 INTRODUCTION TO GENERAL SCIENCE 



is the test for chloride, and water which gives this result with 

 nitric acid and silver nitrate may contain animal matter. 

 Bring a sample of water and test it. Always add the nitric 

 acid first. 



d. Hardness can be indicated by seeing how much soap 

 is necessary to produce a strong lather. To half a test tube 

 of distilled water, add a very little soap solution, cover the 

 open end with the thumb, and shake. See if the lather will 

 last three minutes. If not, add a little more soap solution. 

 Now test a sample of water and see how much more soap is 

 needed to produce the same result. Why is hard water said 

 to be expensive to use? 



197. FOOD PRESERVATION OF FOOD 



Bacteria and molds, as well as other microorganisms, live 

 upon the same food as do man and the other animals. For 

 that reason we must prevent these undesirable organisms 

 from entering our food. This can be accomplished by heat- 

 ing, drying, salting, and smoking the material to be preserved; 

 and, in some cases, a large proportion of sugar will prevent 

 the unfavorable growths. It may be necessary to inclose the 

 food in a bacteria- and ' mold-proof container. 



Coolness and cleanliness will prevent the unnecessary 

 introduction and rapid growth of bacteria. Eggs may be 

 preserved by immersing them in a 10 per cent solution of 

 water glass. 



References : 



1. 1710 : 297-307. Preservation of Food. 



2. 1901 : 139-156. Preservation of Food. 



3. 1901 : 157-168. Use of Preservatives. 



4. 1901 : 169-181. Preservation by Canning. 



