DIKECT EXAMINATION 



43 



3. Substances in which the organisms are too numerous to 

 permit ready counting by means of the 0.004 c.mm. areas of the 

 Thoma-Zeiss hemacytometer. It now becomes necessary to use 

 dilutions, which are made as follows. 



Making the Dilutions. — The dilutions general!)' used are i-io. 

 Rarely will it be found necessary to use higher dilutions. Should 

 this, however, become desirable, a dilution of i-ioo is to be made. 

 The highest counts so far recorded were in the case of two tomato 

 pastes which showed 2,400,000,000 and 4,000,000,000 bacilli per 

 cc. In these instances dilutions of i-io were used and proved 

 quite satisfactory, though it was evident that a greater number of 



Fig. 6. — Thoma-Zeiss hemacytometer. Complete equipment for blood count- 

 ing. This is very convenient for making bacterial counts in catsups, jams, jellies 

 and other vegetable foods and also in animal food substances. 



bacilli per cc. would have necessitated the use of a dilution of 

 i-ioo. However, a dilution of i-io is all that is required for 

 practical purposes, as a bacterial count of 4,000,000,000 and more 

 per cc. would indicate the decomposed condition of the food 

 substance and its unfitness for human consumption. 



In case of liquids and near liquids, 9 cc. of distilled water is 

 added to i cc. of the substance, and in the case of pastes and 

 similar products, 9 cc. of distilled water is added to i gram (or 

 I cc. semiliquid) of the substance. The dilutions are made in 

 25 cc. graduated cylinders, which answer the purpose very well. 

 Or 100 cc. graduates may be used for making the dilutions, adding 



