42 Lectures on- Bacteria. \^ v. 



be easily ascertained by direct observation. It is obvious that 

 the purpose under consideration can be most certainly and most 

 simply effected in the way which has just been described. The 

 result certainly can never be more than approximatively exact, 

 because the nature of the process does not ensure that all the 

 germs capable of development which find their way to the 

 gelatine in the apparatus do in any given case actually develope, 

 or in the case of air-suction that all the germs without exception 

 are always actually caught. No other method which has not 

 this fault in an equal or even greater degree and without the 

 advantage of fixing the germ has up to the present time been 

 devised, nor is it easy to imagine one that would be practicable. 

 It may be added here that Koch's method has the further 

 advantage of making the sorting and selection of Bacteria for 

 isolated culture comparatively easy. Each of the groups derived 

 from a single germ in the experiments above described must 

 contain a single species without admixture. To obtain a 

 quantity of this species for a pure culture we have only to remove 

 a sample from the group with the needle. To sort a mixed mass 

 of Bacteria requires simply the spreading small quantities of it 

 over a large amount of gelatine, and thus isolating germs capable 

 of development. The groups formed from these germs supply 

 pure species-material. Various other experiments have been 

 tried with the same objects and on the same principles, but with 

 less perfect arrangements and methods ; we must not, however, 

 enter here into a more detailed account of them. The most 

 elaborate are those instituted by Miquel and continued from 

 year to year in the meteorological observatory at Montsouris, 

 near Paris, intended especially to ascertain the distribution of 

 germs in the air and in water (15). 



All researches hitherto conducted have given the general 

 result described above, and a further one which might have 

 been expected beforehand, namely, that the number of germs 

 capable of development varies, other conditions being the same, 

 with the place, the time of year, the weather, and other circum- 



