214 THE FLOATING-MATTER OF THE AIR. 



first mentioned were never raised to the boiling tempe- 

 rature at all, the maximum to which they were exposed 

 being some degrees below their boiling-point. The 

 result is that while every one of the four bulbs boiled 

 for ten minutes has become turbid and covered with 

 scum, one only of the ten discontinuously heated bulbs 

 has given way ; nine of them remain as brilliant as at 

 first. 



It is obvious from what has gone before that two 

 hundred and forty minutes might have been substituted 

 for ten minutes without altering this result. Five 

 minutes of discontinuous heating can accomplish more 

 than five hours' continuous heating. 



On the same date three bulbs charged with an acid 

 infusion of London hay were subjected to the same dis- 

 continuous treatment. They all remain brilliant to the 

 present hour. 



On the 7th of February four of Cohn's tubes were 

 charged with turnip-infusion, which was heated discon- 

 tinuously night and morning up to a temperature of 

 205° Fahr. The total time during which they were 

 exposed to this temperature was about three minutes. 

 They were permanently sterilized, and exhibit a singular 

 brilliancy to the present hour. 



The discontinuous method of heating has also been 

 applied with success to the closed chambers. One mode 

 of operation is this : — An oil-bath is heated to a tempe- 

 ratm-e of 300° Fahr. The charged test-tubes of the 

 closed chamber are then plunged in the oil, which clasps 

 the tubes to the level of the surface of the infusion. 

 They are either raised to incipient boiling and then re- 

 moved, or sufiered actually to boil for thirty seconds 

 and then removed. Another mode of heating is this : 

 instead of being plunged into hot oil, the test-tubes are 

 plunged for two or three minutes into boiling water. 



