1 66 THERMOGENIC BACTERIA. 



In many instances the spontaneous heating of the aforesaid 

 vegetable matters may develop into spontaneous combustion, 

 whereby barns and spinning works have often been set on fire. 

 Nothing definite is known of the precise conditions concerned in 

 this phenomenon. In the first place, a knowledge of the igniting 

 temperature of the various substances under consideration is 

 necessary, and as this temperature is probably higher than the 

 maximum heat supportable by living organisms, the actual ignition 

 in such cases cannot be directly attributed to their vital activity. 

 Hence, the probable explanation of spontaneous ignition is that 

 certain micro-organisms, by their oxidising action, convert the 

 vegetable fibres into a humous, porous mass, which is then (like 

 finely divided iron, &c.) capable of occluding oxygen, whereby 

 ignition is induced. More detailed researches into this pheno- 

 menon are still required. 



104. The Spontaneous Heating- of Hops 



is well known to every brewer from wide personal experience. 

 For a more intimate knowledge of the subject we are indebted 

 to J. BEHRENS (I.), who has not only discovered a cause for the 

 phenomenon, but has also traced a connection between the latter 

 and the well-known presence of trimethylamine in hops, first 

 shown by GRIESSMAYER (I.). Behrens found, in hops that had 

 become warm, a fission fungus, in a condition of almost pure 

 culture, which he named Bacillus lupuliperda, and explained to- 

 be nearly allied to the Bacillus fluorescens putidus described by 

 FL^GGE (I.). The cells of this newly-discovered motile microbe 

 are about 0.7 /x in breadth, and vary in length, according to the 

 conditions of cultivation, from 0.7 to 2.5 /x. It liquefies gelatin. 

 Peptone alone is insufficient for its support, a second substance 

 from which it can derive carbon being required ; consequently 

 it belongs to the group of peptone-carbon bacteria established by 

 Beyerinck. It thrives freely in hop extract and quickly renders 

 sugar-free media alkaline by excreting copious quantities of am- 

 monia bases, especially trimethylamine. In presence of sugar the 

 reaction of the medium at first becomes acid, butyric acid being 

 formed, although in Behrens' experiments only to the extent of 

 o.i per cent, at most. This microbe, which gives rise to spon- 

 taneous heating in hops, and causes them to give off an odour of 

 trimethylamine, was found by Behrens in all the samples of hops 

 examined by him. It appears to be chiefly domiciled in the soil 

 and passes thence to the hop cones, which, being fairly hygro- 

 scopic, attract moisture when bagged, and thus enable the bacillus 

 to develop, the hops becoming "warm" and commencing to de- 

 compose, whereby they are reduced in value. Timely prevention 

 may be ensured by removing the bagging in which the hops are 

 packed, and thus admitting cool and dry air into the interior of 



