It is a matter of some difficulty even to demonstrate that the blood is an alkaline fluid. The reason is that its red colour masks the reaction. The plasma itself is, however, practically colourless, and by allowing it to mingle with red litmus solution, after its corpuscles have been strained away, a blue tint may be observed. This was effected by Liebreich, who moistened dry neutral plates of plaster of Paris with litmus solution.