IN ANIMAL TISSUES. HARRIS. 265 



under considerable pressure. A fawn coloured, viscid liquid 

 drips out and is received under toluene. This juice is sub- 

 sequently ground up with powered glass and filtered through 

 two layers of cheese cloth to free it from connective-tissue and 

 the debris of blood-vessels, etc. Some preventative of putre- 

 faction must be used although any such substance reduces the 

 energy of tissue-respiration. 



V. DESCRIPTION OF A TYPICAL OBSERVATION. 



Three cubic centimeters of absolutely fresh press-juice 

 prepared as just described, were shaken in a test-tube with 

 10 c.c. of 0.05% solution of soluble Prussian blue at room 

 temperature (about 17 C). The blue colour began to disap- 

 pear immediately, and in less than a minute after passing 

 through light blue, light green and greenish grey, the mixture 

 became light grey in colour. No trace of pigment remained. 



When the same volume of boiled juice was used, no 

 decrease in the intensity of the blue colour of the solution was 

 observed at the end of several hours. The reducing activity 

 of the juice was found to diminish somewhat rapidly with time. 

 With a mixture containing 3 c.c. of the press-juice 24 hours 

 old, and 10 c.c. of 0.05% Prussian blue solution, it was found 

 that ten minutes elapsed before its colour became green grey, 

 and two hours before it became completely colorless, (grey). 



VI. EXAMINATION OF POSSIBLE FALLACIES. 



Since the change from the coloured to the leuco condition 

 is the sign of reduction having taken place, one must guard 

 against confusing the fading of pigments through reduction 

 with fading from causes other than bio-chemical reduction. 



(a) The earliest criticism offered was that the fading of 

 the Prussian blue was due to the presence of "alkaline salts". 

 Now free alkali, which undoubtedly fades Prussian blue, does 

 not exist in the tissues or their juices. The inorganic salts of 

 tissues and tissue-juices do not bring about any fading of 

 soluble Prussian blue. 



