We know that the color of the body’s skin and mucous membranes changes with the color of the blood stream. The red color of blood is due to the hemoglobin contained in red blood cells. When hemoglobin is sufficiently combined with oxygen to become oxyhemoglobin, it is bright red in color; when it has released oxygen and becomes deoxyhemoglobin, the color changes to dark red. The blood in arteries and capillaries contains more oxyhemoglobin and less deoxyhemoglobin, so it has a bright red color, and the red color is still obvious through thin mucous membranes and translucent nails. Aniline, nitrobenzene