Fåhræus (1921), a Swedish physician who devised the erythrocyte sedimentation rate, suggested that the four humours were based upon the observation of blood clotting in a transparent container. When blood is drawn in a glass container and left undisturbed for about an hour, four different layers can be seen. A dark clot forms at the bottom (the "black bile"). Above the clot is a layer of red blood cells (the "blood"). Above this is a whitish layer of white blood cells (the "phlegm", now called the buffy coat). The top layer is clear yellow serum (the "yellow bile").