28 THE NA Tl RE-STUD Y RE VIE W [a . l - ) ^ v ^-, , 9 o& 



is possible that some germs from the air may fall into your broth. 

 You must therefore sterilize the tubes and the broth in them again. 

 It will not do to place them in the oven as you did before because you 

 would dry the broth up, and crack the tubes. You must employ a 

 method of wet sterilization which you can easily do by setting the tubes 

 in a tumbler in a steamer and steaming them for thirty minutes. One 

 steaming is not sufficient, however to kill all the germs. You must 

 steam them thirty minutes today, thirty minutes tomorrow, and thirty 

 minutes the next day. This is called discontinuous sterilization. 

 After the third clay the broth is completely sterilized and there is no 

 living thing in it. The broth in all of the tubes should then look just 

 alike. 



Now for the experiment. You may set tube No. i away without 

 opening it at all. From No. 2 you may take out the stopper, keep 

 it out one minute, then replace it. From No. 3 you may remove 

 the stopper and leave it out thirty minutes. From No. 4 remove the 

 stopper then pick up a little dust on the point of a knife and drop it 

 into the tube, not enough to be visible at all after it is put in, but 

 just a little bit. To No. 5 add dust just as you did in No. 4, 

 and then close the tube and set in a pan of boiling water and leave 

 it there 10 minutes. To No. 6 add dust just as you did in No. 4 and 

 also add a little carbolic acid. To 7 add a drop of ordinary drink- 

 ing water. Now set the tubes all away together in a moderately 

 warm place, and look at them day by day for a week or two. After 

 a time you will notice that Nos. 3, 4, and 7, and possible No. 2 are 

 becoming cloudy, while No. 1, 5 and 6 do not become cloudy. A 

 few days later this cloudiness increases. Possibly a scum appears 

 upon the surface and very likely a sediment in the bottom of tubes 

 Nos. 3, 4, and 7. If you remove the stoppers from 3, 4 and 7 and 

 shake them you will find that they smell badly. Nos. 2,5, and 6 do 

 not smell so. In these turbid tubes the broth is really decaying. In 

 the clear tubes it is not. If we should examine the contents of these 

 tubes with the microscope, we would find in the turbid tubes myriads 

 of wiggling, living, squirming germs, bacteria. In the clear tubes 

 there are none of these. 



Now let us see what all of this means : It is quite clear that in 

 tube No. 1 we killed all the germs and since we did not open it.no 

 germs could get in. It is equally clear that in tube No. 4 we added 

 some dust and there was a great possibility that germs were present 

 in the dust. Indeed it would have been possible to have found them 



