r 



178 



There seems to be no important difference between the two anti- 

 septics 



At first the agglutination lest was carried out at 37° and read 

 8 hours later. From orientating experiments at different tempe- 

 ratures it appeared however to progress distinctly more rapidly 

 and also to proceed further, round about 50° and therefore this 

 temperature was employed in the major portion of the experi- 

 ments. The temperature could vary a couple of degrees, excep- 

 tionally as much as 5° degrees on either side. At first the tubes 

 were given 3 hours at 50°, but as it was found that in this in- 

 terval the reaction did not always reach its final point the time was 

 afterwards lengthened by an hour. There is nothing to indicate 

 that these discrepancies can have had any important influence on 

 the principal facts of the investigation and therefore they will not 

 be further discussed in the description of the individual experiments 



The method used for standardising the bacterial suspensions 

 must be discussed a little further. J} 



The principle of using the density through which an object 

 can just be distinguished, as a basis of the determination of the 

 density of a bacterial suspension, has been used by several other 

 authors but usually in a more complicated form. In my experience 

 however, a degree of accuracy can be attained by this very simple 

 melbod which is sufficient for the large majority of serological tests, 

 and moreover it is difficult to do better with other methods. 



The following experiments illustrate the accuracy of the method. 

 A dense bacterial suspension was prepared. Variable quantities were 

 measured into test tubes of 14— 16 mm. internal diameter. This 

 diameter was correct to within 1/2 mm. for each tube. With a 

 graduated pipette salt solution was now added until the letters could 

 just be read through the suspension. (During the process the in- 

 vestigator was ignorant of the amount of bacterial suspension which 

 had been measured into the tube). If the volume of the concen- 

 trated bacterial suspension is V l5 the added salt solution, V 2 , and 

 the diameter of the tube d, the concentration* is given by the equation, 



V l+ V 2 15 



A — V j d 



The last factor is based on the assumption that the concentra- 

 tion through which the letters can just be read is inversely pro- 

 portional to the diameter of the tube, which is only approximately 

 correct. 



The experiments were made partly by myself and partly by a 

 colleague who was used to estimating the concentration of bacterial 

 suspensions by comparison with standard suspensions, but who had 

 not used the technique described here before and only had it 



* in a provisional, arbitrary unit. 



