360 THE IRON BACTERIA. 



thread bacteria, the ferric oxide being deposited in their sheath- 

 ing in the same way that silica is accumulated in the plates 

 of the diatoms. We are indebted to S. WINOGRADSKY (IV.) for 

 proving the correctness of this view, and for refuting the opinion 

 of Zopf that the deposition is purely mechanical ; and we have to 

 thank the same observer for the more intimate investigation of the 

 process in question. 



The species Crenothrix polyspora, Cladothrix dichotoma, Lep- 

 tothrix ochracea, &c., occur in particular abundance in such 

 waters as are rich in iron, not in the form of oxide, but as the 

 soluble bicarbonate of the protoxide, FeH 2 (C0 3 ) 2 . Ferruginous 

 springs, ascending from the deeper strata of the rocks, bring up 

 this substance in a ready-formed state ; and in the water of the 

 upper strata it is produced by the decomposition of vegetable 

 matter, the iron, both in this and in the water itself, being con- 

 verted during cellulose fermentation into the hydrocarbonate. 

 This compound is then absorbed by osmosis into the bacterial 

 cell, where it is split up by the plasma and oxidised, according to 

 the equation 



= Fe 2 (OH) 6 + 2CO 2 . 



The ferric oxide is then stored up in the sheath, to which it 

 imparts a coloration, initially pale yellow but gradually changing 

 to dark brown. Freshly precipitated ferric hydroxide is, as we 

 know, somewhat soluble in water, but afterwards gradually passes 

 into a condition in which it is only attackable by weak acids. 

 This change can be traced in the young bacteria, the colouring 

 matter in the yellow sheath being at first extractible by washing 

 with water containing C0 2 in solution. Subsequently, however, 

 dilute hydrochloric acid must be resorted to, and at a still later 

 stage even this solvent is powerless to extract the brown deposit. 

 A very fine and fast blue stain can be produced in young sheaths 

 (the iron in which is still soluble in acid) by exposing them to 

 a mixture of hydrochloric acid and yellow prussiate (potassium 

 ferrocyanide), whereby the hydroxide is dissolved, immediately 

 converted into Berlin blue, and re-precipitated. In older threads 

 the deposits of ferric oxide increase to a thick incrustation, and 

 entirely conceal the structure of the cells. 



Winogradsky discovered that these bacteria thrive only when 

 ferrous carbonate is available, and that growth is arrested directly 

 the nutrient medium contains no iron, or only iron in the condi- 

 tion of oxide. This fact entails the conclusion that the life of 

 these bacteria is mainly sustained by the energy liberated during 

 the oxidation of ferrous oxide to ferric oxide. Consequently, these 

 organisms rightly deserve their name of "iron bacteria." According 

 to the discoveries of H. MOLTSCH (I.), iron can be replaced in this 

 oxidation process by the chemically allied metal manganese. These 

 bacteria require but a very small quantity of other nutrient 



