SULPHUR BACTERIA 



Methods of Investigation. 



Our knowledge of the sulphur bacteria has been built up 

 by the application of three methods of examination, each of 

 which has its advantages and disadvantages. 



1. Direct Observation under Natural Conditions. — This 

 is the best method of observing the developmental life-his- 

 tory of these bacteria, provided that the organisms can be 

 recognized with certainty, and readily distinguished from 

 all other organisms occupying the same field. The sulphur 

 bacteria are then observed in the normal course of their 

 development under natural conditions, and in competition 

 w^ith other organisms of the same class. The sulphur in- 

 clusions and the purple colour are noticeable features which 

 make their identification easy, and facilitate the identification 

 of the passage of an organism from one phase of development 

 to another. When observations of the same mass-cultures 

 are conducted at periodical intervals throughout the whole 

 year, the course of the life-history of an organism is sometimes 

 easy to follow. Also, in such cases it is often possible to 

 observe in organic connection two different forms of growth 

 which, without the observation of such connection, would be 

 regarded as specifically distinct. The valuable observations 

 of Zopf on both the coloured and colourless sulphur 'bacteria 

 were obtained by this method, and the same applies to the 

 published observations of the majority of the other mor- 

 phologists who have studied these bacteria. This method is 

 particularly valuable when the organism under observation 

 is present in very large numbers, as is usually the case in mass- 

 cultures of the sulphur bacteria. 



2. Glass Slide and Coverslip Observations. — This method was 

 favoured by Winogradsky (1-3), and is described in Chap. IV. 

 In essentials it consists in placing material on a glass slide and 

 observing it at periodic intervals, so that the same individuals 

 are viewed after sufficient time has elapsed to enable them^ to 

 advance in growth. This method is valuable when it supplies 

 positive results, but no deductions are permissible from the 

 negative results obtained by its use. If negative results are 



