STUDIES ON FER^IENTATION". 85 



in it by the common air, and these, again, as regards their 

 nature and number, are dependent upon the situation of the 

 vessel containing them, its height above the ground, the 

 time of year, the disturbance of the atmosphere, and other 

 causes. 



The fortuitous association of other forms, in growths which 

 we believe to be uniform and independent, constitutes one of 

 the principal difficulties that occur in the study of the lower 

 organisms, particularly that of fungoid growths. The fact that 

 the germs of many of these little beings exist in the atmo- 

 sphere in the form of dust, invisible to the naked eye, or, 

 as such, spread over the surface of the diiferent matei"ials and 

 objects used in experiments, exposes the student to constant 

 risk of wrongly interpreting the results which, come under 

 his notice. He has sown a plant, and is observing the course 

 of its development. Without his knowledge, spores of another 

 plant have got mixed with his growth, and germinated. In 

 his ignorance, he will attribute all that he sees, all the changes 

 Avhich he describes and which he sketches, and all the con- 

 clusions which he draws, to the one plant which engages his 

 attention. If he is dealing with bacteria, vibrios, and, generally 

 speaking, the infinite variety of mobile microscopic organisms, 

 his embarrasment will be greater still. Again, inasmuch as 

 the medium which serves as substratum for growths has a 

 considerable influence on the fertility of the germs in contact 

 with it, as well as on their ulterior development, it often 

 happens that germs deposited fortuitously by the particles of 

 dust which fall from the atmosphere or collect on objects are 

 fertile and multijaly with rapidity, whilst those which have 

 been directly sown, no matter in what number, remain sterile, 

 or multiply very slowly. If we place in a young wine some 

 mycoderma aceti, we shall obtain mycoderma vini; by placing 

 some mycoderma mni in an old wine, especially if it is a little 

 acid, we shall obtain mycoderma aceti* It is from facts of 



* See, on this subject, the author's Etudes sur le Vinaigre, Paris, 1868, 

 p. 76, note ; and esj)ecially Etudes sur le Vin, 2nd Edition, 1873, p. 19. 



