i8 



LABORATORY EQUIPMENT 



flask. The fluid to be filtered is placed into the glass cylinder and sucked through into 

 the flask by means of a vacuum produced here. For the purpose of collecting very 

 small quantities a test-tube may be placed into the exhaust flask (Fig. 12). 



Preparation of Dilutions and Measurement of Small Amounts of 



Bacteria. 



All serological methods are to be considered on quantitative bases. In 

 serum diagnosis as well as in the therapy, the amount of the serum- employed 

 is the deciding factor. Similarly, the number or amount of bacteria required 

 either for the purposes of immunization or serological reactions is of ex- 

 treme importance. 



One cubic centimeter is the unit of measure for serum and all fluid mate- 

 rial (Bouillon cultures, exudates, etc.). // small quantities are required, it 

 is best to dilute the fluid with 0.85 per cent, saline solution. The exact prepa- 

 ration of dilutions is one of the most essential technical procedures in serum 

 diagnosis. Some general rules may be of help. 



1. Never should amounts less than o.i c.cm. be measured out directly. 

 For beginners even o.i is best measured in the form of a dilution, as errors 

 are apt to occur very easily. 



2. The decimal system should be adhered to as much as possible. 



3. The dilution should be made just before it is to be used, inasmuch 

 as many substances retain their activity best, or only, in concentrated 

 form. 



The following is an example of correct forms of dilutions: 



The stock dilution of i : 10 is made by measuring off i c.cm. of toxin and adding 

 9 c.cm. of 0.85 per cent, of saline. 



The dilution i : 100 can be made by taking i c.cm. of toxin and adding 99 c.cm. of 

 saline. It is more practicable, however, to take i c.cm. of the i : 10 stock dilution 

 and add 9 c.cm. of saline. If the dilution i : ib is not present and only a small amount 

 of the dilution i : 100 is desired, the latter is made by taking o.i toxin : 10.0 NaCl sol. 

 Similarly 1:1000 = 0.1:100=1 c.cm. of the dilution (i : 10) :ioo=i c.cm. of the 

 dilution (i :ioo) :io.o. 



