246 MANUAL OF BACTERIOLOGY. 



principles. Great care is necessary to arrive at correct conclu- 

 sions, particularly in the case of animal tendons. In many in- 

 stances quite stable compounds are formed between tendon and 

 germicide, and living organisms may- be so imbedded in such 

 a substance that subsequent growth in a test-cuture is impos- 

 sible. The use of a suitable inhibitor, and, prior to final 

 culture-tests, a prolonged soaking in sterile water, will promote 

 the accuracy of the results. 



So many and often such obscure chemical and physical fac- 

 tors enter into the action of chemical germicides that uniform 

 results are not possible within narrow limits. This accounts 

 for the conflicting results obtained by different investigators, 

 and even the same investigator at different times. A number 

 of variable and only partially controllable conditions enter into 

 every test. Results with gaseous disinfectants are especially 

 uncertain on this account. No gaseous disinfectants have 

 any great power of penetration, and consequently act only 

 where the bacteria are freely exposed and then not always 

 with certainty. 



Physical Disinfectants.— Drym^.— The effect of drying 

 differs with different organisms and also with the same organ- 

 ism depending upon whether there are spores present or not. 

 The vegetative cells of some organisms are very readily killed 

 by drying, but spores, on the contrary, are not affected in this 

 way. Spores have, in fact, been preserved for years dried on 

 silk threads. 



Agitation. — Bacteria are killed by prolonged violent agita- 

 tion. They may be actually shaken to pieces. But ordinary 

 gentle shaking" appears in some cases at least to stimulate 

 growth. 



Cold. — Bacteria are very resistant to cold, those that have 

 been so tested survive the low temperature of hquid air, 

 though of course there is no growth at this temperature. The 

 organisms subjected to this low temperature grow readily 



