METHODS OF INVESTIGATION 11 



The standard Winkler method and its modified forms 

 require rather hirge samples of water; from 125 to 350 

 cc. are commonly used for a single analysis. In some 

 types of investigation only smaller samples may be 

 available. One should then employ micro modifications 

 of the original procedure. Krogh (1935) used small 

 water bottles containing but 7 to 15 cc. of water and 

 van Dam (1935) described a method requiring only 1 cc. 

 This latter procedure was further improved by Fox and 

 Wingfield (1938) to whom we refer the reader for details. 



Krogh (1935) has pointed out that all forms of the 

 Winkler method have two sources of error in common. 

 Both are of special importance for the problem under 

 consideration here since they become especially noticeable 

 when the oxygen tension of the water is low. "One is 

 the diffusion of oxygen that takes place whenever the 

 sample is not in equilibrium with the atmosphere, and 

 the other is the uncertainty of the correction for oxygen 

 dissolved in the reagents." Krogh (1935) has worked 

 out a micro modification of Winkler's method in which 

 these possibilities of error have been eliminated. He 

 collects the water sample and carries out the reaction in 

 one and the same syringe pipette of 10 cc. capacity. The 

 method gives variations of only =t 0.007 cc. of oxygen per 

 liter and deserves to be used more widely. It might be 

 possible to modify it further so that it could be applied 

 also to impure waters. 



C. Parasitic habitats. Some parasites occur in a gas- 

 eous environment, for example, in the intestine of ani- 

 mals, where they are surrounded by the intestinal gases, 

 or in the swim-bladder of fishes or in the lungs of va- 

 rious hosts, etc. The analytical procedures are then 

 obviously the same in principle as those employed in 

 the case of terrestrial habitats. Due to the fact that 

 most of the time only small amounts of gases can be se- 

 cured, it will often be necessary to use micro gas analysis 

 methods. 



