PURIFICATION OF LITMUS. 509 



A proper bacteriological examination must take into account 

 the several possibilities just mentioned. Inasmuch as anaerobic bac- 

 teria may be the cause of such food infection, plates should also be 

 made in glucose gelatin and in glucose agar and allowed to develop in 

 an anaerobic apparatus (p. 314). The gelatin plates are, of course, 

 developed at ordinary room temperature, whereas the agar 

 plates should be grown at 37. The several varieties of colonies 

 should be transplanted and the effects of the pure living cultures, and 

 also of the filtered cultures, should be studied on animals. The animal 

 employed should, if possible, be susceptible N to the poisonous food 

 itself. In many cases, the dog and cat are preferable to the ordinary 

 experimental animals. 



At the time the plate cultures are planted bouillon tubes should 

 be inoculated and allowed to develop, some at 37 and some at ordinary 

 room temperature. The cultures, thus obtained, can be tested by 

 subcutaneous injection into guinea-pigs or rabbits. In case of the 

 death of the experimental animal, the serous fluid in the large cavi- 

 ties and especially the heart-blood should be used to obtain pure 

 cultures of the pathogenic organism which is present. 



Purification of Litmus. 



The ordinary litmus cubes, as met with commercially, 

 do not yield a pure blue solution of the pigment. This is 

 due to the presence of various impurities, notably of a red 

 pigment. A pure blue litmus solution can be readily pre- 

 pared according to the following directions: 



25 g. of litmus are placed in a beaker and 250 c.c. of 

 distilled water are added. The solution is then heated on 

 a boiling water-bath or in steam for about 30 minutes. The 

 deep blue liquid is then decanted; water is added to the 

 residue and the mixture heated as before, and finally the 

 blue solution is again decanted. This washing out of 

 the residue is repeated once or twice. The combined 

 aqueous extracts are allowed to settle over night and are 

 then filtered. The filtrate is concentrated in an evaporat- 

 ing dish, over a flame, to less than one-half. It is then 



