262 AGRICULTURE ON THE PACIFIC SLOPE 



making this test get a little pure milk and add formalin 

 to it to see how little formalin it takes to produce the 

 purple color. 



We notice that the acid curdles the milk; in the same 

 way the acid made by the bacteria curdles the milk. 

 When there is much acid in the milk, it acts as a 

 poison (toxin) to the bacteria and stops their growth. 

 If we add limewater, which combines with the acid and 

 takes away the sour taste, the bacteria can grow again. 



Toxins and antitoxins. When disease bacteria, such 

 as those of typhoid, get into our bodies, they produce 

 substances which are poisonous to us (toxins), and our 

 bodies then make " antitoxins " to combine with tlie 

 toxins and make them harmless. But it often happens 

 that the body cannot do this fast enough, and we must 

 then take antitoxins from another animal which has 

 had the same disease, and put them into our own blood 

 to help in overcoming the poison. 



Prevention of bacterial diseases. One way to pre- 

 vent disease is to do away with all garbage heaps and 

 everything else that is not clean. Stables spread dis- 

 ease because they are breeding places for flies. Catch 

 a fly and put it into one of the flat bottles shown in 

 figure 153. Let it crawl about on the gelatin for a 

 short time and then escape. Put the cotton back into 

 the bottle and let it stand a few days. Do you see any 

 bacterial growth in the footprints of the fly (see figure 

 155) ? When you think where flies came from when 



