FERMENTATIONS WITH NO SOLUBLE FERMENTS 209 



which changes the starch into sugar, and then converts the sugar into 

 butyric acid. 



Altogether, about twenty species of butyric-acid bacteria have been 

 described, but some of them are undoubtedly mixtures of different 

 species, so that we cannot say at present how many distinct species are 

 in existence. 



IV. Fermentations involving changes in the nitrogenous organic 

 compounds of the soil. In this group may be conveniently placed the 

 series of changes which result from the activity of the soil bacteria, as 

 a result of which ammonium compounds, and subsequently nitrites and 

 nitrates, are produced. Also, in this group may be included those 

 activities that result in the formation of oxides of nitrogen or of free 

 nitrogen. The nature of these changes have already been described 

 (see last chapter). 



V. Fermentations involving the transformation of sulphur com- 

 pounds. As one of the results of the activities of different kinds of 

 bacteria which decompose organic matter, sulphuretted hydrogen is 

 formed. The sulphur-bacteria absorb this compound, and free sulphur 

 is formed, which is later changed by them into the sulphate. We may 

 also include in this group the changes effected by other bacteria which 

 change the sulphates into the sulphite form. 



VI. Propionic-, citric-, and oxalic-acid fermentations. A bacillus 

 has been described which transforms lactic acid into two parts of 

 propionic and one part of acetic acid, carbon dioxide and water being 

 formed at the same time. Certain fungi also are known which bring 

 about the formation of citric acid as a chief product. Two of these, 

 known as Oitromyces Pfefferianus and Citromyces glaber respectively, 

 belong to the mould fungi. They produce citric acid from glucose, 

 and under certain circumstances the amount of acid formed may be 

 more than 50 per cent, of the glucose employed. Other fungi, notably 

 Penicillium and Sclerotinia, are known to change sugar into oxalic 

 acid, and this power is possessed also by certain yeasts, e.g. Saccharo- 

 myces Hansenii, by the aid of which large quantities of calcium oxalate 

 may be formed. 



