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%th Aprils 1892. 



Professor FitzGerald, B.A., C.E., in the Chair. 



Allan P. Swan, Esq., F.L.S., read a Paper on 

 "MILK AND ITS FERMENTS." 



Mr. Swan said : — Perhaps no branch of natural science has of 

 late years made more real progress than that which relates to the 

 biological study of ferments and micro-organisms. The theories 

 on fermentation which were worked out by Pasteur thirty years 

 ago have gradually revolutionised all pre-existing ideas on the 

 subject, and placed the study of ferments on a firm basis, while 

 the ingenious methods of research more recently introduced by 

 Koch are, I think, only second in importance to Pasteur's 

 wonderful discoveries, as they have overcome difficulties of 

 manipulation in the study which were formerly insurmountable. 

 In this paper I only purpose to try to explain some of the 

 natural and apparently spontaneous chemical changes which 

 can occur in milk, and which are always caused by ferments. 



We understand fermentation to be a phenomenon indirectly 

 due to the life of micro-organisms, and caused by a special force 

 which is derived from them. These organisms, when present 

 in a liquid which is favourable to their life, grow and multiply 

 rapidly, and their development is generally accompanied by 

 chemical changes in the liquid itself, which changes are due to 

 a disturbance in its constituents, causing them to become 

 rearranged in other combinations. Fermentation may be said 

 to be of two kinds, natural and artificial. When milk turns 

 sour in spite of all we can do to prevent it, the change may be 

 said to be caused by a natural lactic fermentation ; if, on the 

 other hand, we obtain the lactic ferment by collecting a mass 

 of its living organisms, and add them to fresh milk in order to 



