138 THE FLOATING-MATTER OF THE AIR. 



wards. Sixty, eighty, and even one hundred and 

 twenty minutes' boiling were found in some cases in- 

 sufficient to sterilize the infusions. One marked dif- 

 ference, however, exists between Dr. Eoberts and Pro- 

 fessor Cohn. The former found five minutes' boiling 

 sufficient to sterilize unneutralized hay-infusion, but 

 one, two, and even three hours' boiling insufficient to 

 sterilize superneutralized hay-infusion ; while the latter 

 noticed no difference of this kind, but found acid and 

 neutral infusions equally resistant.^ 



§ 3. Hay-infusions, Freliminary Experiments 

 with Pipette-bulbs. 



1 have now the honour to submit to the Koyal 

 Society an investigation which embraces among others 

 the points here referred to, and which has proved far 

 more difficult and laborious than I expected it would be. 



On the 27th of September, 1876, a quantity of 

 chopped hay was digested for three hours and a half in 

 distilled water maintained at a temperature of 120° Fahr. 

 The infusion was afterwards poured off, and its specific 

 gravity reduced to the exact figure given by Dr. Eoberts, 

 viz. 1006. It was then filtered and slightly super- 

 neutralized. Precipitation occurred on the addition of 

 the potash, and the infusion was boiled for five minutes 

 to render the precipitation complete. It was then re- 

 filtered, and introduced into a series of bulbs of the 

 same size and character as those described by Dr. 

 Eoberts, and called by him ' plugged bulbs.' ^ 



Each bulb was a cylinder about four inches high and 

 upwards of an inch wide, with a long neck attached to 



' ' Ein constanter Unterschied in der Zeitdauer zwischen sauren 

 und neutralen Aufgiissen, wie ihn Roberts gef unden, trat in unseren 

 Versuchen nicht hervor ' (p. 259). 



2 Phil. Trans, vol. clxiv. p. 460. 



