PUTKEFACTION AND INFECTION. Ill 



may be overridden by other causes. Every one of the 

 other tubes containing the hay-infusion showed spots of 

 mould upon the clear liquid. 



Turnip. — Four of the thirty-five tubes were very 

 muddy, two of them being in the row next the stove,, 

 one four rows distant, and the remaining one nine rows 

 away. Besides these, seven tubes had become clouded- 

 There was no mould on any of the tubes. 



Beef. — One tube of the thirty-five was quite muddy, 

 in the seventh row from the stove. There were three 

 cloudy tubes, while seven of them bore spots of mould. 



As a general rule organic infusions exposed to the 

 air during the autumn remained for two days or more 

 perfectly clear. Doubtless from the first germs fell 

 into them, but the germs required time to become or- 

 ganisms. This period of clearness may be called the 

 ' period of latency ; ' and, indeed, it exactly corresponds 

 with what is understood by this term in medicine. 

 Towards the end of the period of latency the fall into a 

 state of disease, if I may use the term, is comparatively 

 sudden ; the infusion passing from perfect clearness to 

 cloudiness more or less dense in a few hours. 



Thus the tube placed in Mr. Darwin's possession 

 was clear at 8.33 a.m. on the 19th of October, and 

 cloudy at 4.30 p.m. Seven hours, moreover, after the 

 first record of our tray of tubes, a marked change had 

 occurred. For the purpose of comparison the second 

 record, fig. 8, is placed beside the first. The change 

 may be thus described: — Instead of one, eight of the 

 tubes containing hay-infusion had fallen into uniform 

 muddiness. Nineteen of them had produced Bacterial 

 slime, which had fallen to the bottom, every tube con- 

 taining the slime being covered by mould. Three tubes 

 only remained clear, but with mould upon their sur- 

 faces. The muddy turnip-tubes had increased from four 

 to ten ; seven tubes were clouded, while eighteen of them 



