PREFACE 



EXPERIMENTAL studies on the micro-organisms readily lead 

 to practical problems relating on the one hand to medicine 

 and on the other to industry. The theoretical and practical 

 problems in this field go hand in hand, and are frequently 

 inseparable. This has also been the case with my investi- 

 gations, as is seen in the first of them which appeared in 

 1878, and still more distinctly in the series published since 

 1 88 1 under the common title 'Recherches sur la physiologic 

 et la morphologic des ferments alcooliques.' Some of my 

 researches are mainly of theoretical interest, whilst others 

 have a more direct practical bearing, and according to 

 whether the one or the other side predominates, they acquire 

 importance for one or the other of the two classes of readers 

 for whom they are written namely, scientific investigators 

 who look for theoretical deductions, and practical men who 

 wish to work in accordance with rational principles and 

 thereby to obtain a material gain. These considerations 

 induced me to publish my investigations in two series since 

 1888, the theoretical studies appearing, as before, under the 

 title given above, whilst those having a direct practical bear- 

 ing were published in a new series. 



The investigations brought together in this book treat in 

 the main of the great questions of the circulation in nature of 



