PREPARATION OF DILUTIONS. 

 The following table shows the details of various dilutions: 



Dilution i : 10. 



Dilution i : 100. 



Dilution i : 1000. Dilution i : 10,000. 



In carrying out these dilutions, it is best to measure off the small quantities o.ii c.c. 

 with a pipette; allow this to run into a well-graduated measuring glass and add enough 

 saline to make the required dilutions. For example, if 30 c.c. of a dilution i : 100 is 

 desired, .3 c.c. should be measured off with a pipette and allowed to flow into a 50 or 100 

 c.c. graduated cylinder and saline solution added up to 30.0 c.c. 



It should always be one's aim to get along with small quantities of the substance to be 

 diluted. If, for example, 8 to 10 c.c. of a toxin dilution i : 100 are required o.i c.c. of 

 toxin+9.9 c.c. of saline should be taken and not i c.c. of toxin and 99 c.c. of NaCl sol. 



Before making any dilution one should always calculate the total amount of substance 

 required; as for example in the following experiment: 



1. Animal o. i c.c. 



2. Animal 0.05 c.c. 



3. Animal o.oi c.c. 



4. Animal o.ooi c.c. 



Toxin subcutaneously 

 Toxin subcutaneously 

 Toxin subcutaneously 

 Toxin subcutaneously 



Here, the total quantity of toxin necessary, is found by adding, to be 0.161 c.c. This 

 represents the minimum amount. It is always advisable to make an allowance for some 

 loss and at the same time bring up the amount to a round or even number. 0.2 c.c. of 

 toxin would fulfill all these requirements. This amount is measured off by a pipette, 

 placed into a graduated cylinder and saline added up to 2.0 c.c., making a dilution of 

 i : 10. Then 0.2 of this dilution (i : 10) is taken, placed into another graduate, and 

 again diluted with saline up to 2.0 thus making a dilution of i : 100. The above problem 

 therefore, of injecting the various animals, can be completed as follows: 



i. Animal receives i c.c. of dilution i : 10 

 i. Animal receives 0.5 c.c. of dilution i : 10 



3. Animal receives i c.c. of dilution i : 100 



4. Animal receives o. i c.c. of dilution i : 100 



The unit for measuring the amount of bacteria grown upon a solid me- 

 dium is represented by a standard sized loop. This platinum loop takes up 



