METHOD OF OBTAINING METHJSMOGLOBIN CRYSTALS. 203 



the defibrinated blood of the animal, adding an equal number 

 of drops of nitrite of amyl in a test-tube, and shaking the 

 mixture vigorously for a minute or two. The colour changes 

 to the dark chocolate tint of niethsemoglobin, and spectro- 

 scopic observation shows the typical absorption bands of that 

 compound. A drop of this liquid is then placed 1 on a slide 

 and covered ; in a few minutes crystals form, which observa- 

 tion with the spectroscope shows to be composed of methaemo- 

 globin. The edges of the cover-glass may then be sealed, and 

 the crystals kept unchanged for several months. 



The crystals obtained from guinea-pig's blood by this 

 process are tetrahedra, which differ only in colour and spec- 

 troscopic appearances from those of oxyhaemoglobin from the 

 same animal. 



The crystals obtained from squirrel's blood are perfectly 

 regular hexagonal plates, which remain dark between crossed 

 nicols. 



The crystals obtained from rat's blood are also perfectly 

 regular hexagoual plates, which remain dark between crossed 

 nicols, and which consequently are precisely similar to those of 

 squirrel's methsemoglobin. This remarkable fact helps to show 

 that the difference between the oxyhaemoglobin of these two 

 animals cannot be a very deep or essential one. 



In the case of rat's methaemoglobin there were, in addition 

 to the hexagons, a few other plates of various shapes scattered 



x i 



A B CD 



A. Regular hexagon. B. Equilateral triangle, c. Intermediate stage between 

 A and b. d. Parallelogram. 



1 This must be done immediately after the formation of the chocolate- 

 coloured liquid ; as in about a quarter of an hour the whole liquid sets into 

 a gelatinous mass of the same colour, from which no crystals are obtainable. 



