CH. v.] traube's cell. Ill 



which then appears as a double black contour with red- 

 purple between. The free end of the glass rod should 

 rest on some guiding support which will keep it in 

 focus but allow it to be stretched. If the observer 

 looks down the microscope while the rod is steadily 

 pulled, the colour of the centre of the thread will 

 be seen to change distinctly, the nature of the change 

 depending on the amount of traction exerted upon the 

 filament : on releasing it the purple colour reappears. 



Compression should give a different colour-change but 

 this cannot be easily exhibited. Other transparent bodies 

 such as gelatine films show similar effects. We have 

 thus seen that both crystalline structure and internal 

 tensions may be accountable for the anisotropism of 

 organised bodies. 



(142) Tranbe's artificial cell ^. 



Traube's method is of great interest as a graphic way 

 of demonstrating the possibility of pressure arising 

 osmotically inside a cell. The method is moreover 

 capable of giving results of great value, especially as 

 modified by Pfeffer''. The following experiment is merely 

 meant to serve as a demonstration. 



Fill a beaker with a solution (2 or 3 per cent.) of 

 potassium ferrocyanide and drop into it a crystal of copper 

 sulphide. The sulphide is instantly coated with a pre- 

 cipitated membrane of copper ferrocyanide. 



In the artificial cell so produced osmotic pressure 



' Traube in Archiv fiir Anatomie und Physiologie (Beiehert and Du 

 Bois-Eeymond), 1867. 



' Osmotische Untersuchungen, 1877. 



