166 THE FLUIDS. 



increased tendency to sink, since, on sinking, they necessarily apply 

 their broad sides to each other. 



2. The precise color of the blood, as a whole, depends very much 

 on the shape of the blood-corpuscles, and therefore indirectly upon 

 physical conditions, some of which have been already mentioned. 

 While the corpuscles retain the biconcave form, their color is 

 lighter; when distended by endosmosis so as to become biconvex, 

 they disperse the light more, and thus render the blood, as a whole, 

 darker. Hence all substances abstracting water from the blood-cells 

 without decomposing them, and thus rendering their central de- 

 pression more distinct, produce in it a bright red to a vermilion 

 color as all neutral alkaline salts, solutions of sugar, &c.; while 

 those agents which render them more nearly spherical as water, 

 ether, and dilute organic acids give to the blood a dark bluish-red 

 color. Still, the form alone does not always determine the color ; 

 for the blood-cells of the amphibia are naturally biconvex, and can- 

 not become biconcave, and yet they are colored bright red by the 

 solutions of sugar and of the salts mentioned above. The following 

 salts produce a vermilion color in the blood of the mammalia, of 

 birds, and of the amphibia : sulphates of potash and soda, nitrates 

 of potash and soda, chloride of potassium, phosphate of soda, car- 

 bonate and bicarbonate of soda, ferro-cyanide of potassium, borax,' 

 iodide of potassium, sulpho-cyanide of potassium, sal ammoniac, 

 sulphate of magnesia, &c. 



But the thickness of the cell-membranes, and their amount of folding, 

 must also influence the color of the blood. If the cells are collapsed, 

 the membrane is thicker and folded ; if distended, they are thinner 

 and smooth. In the latter case, the coloring matter shines through 

 in its natural hue, which is a very dark red ; as in a thin milk-glass 

 a dark red fluid still appears so, but in a thick one, light red. Hence 

 the darker color, usually, of venous blood, and the fact that all sub- 

 stances which corrugate the cell-membranes render the blood dark 

 red, as acetic acid, the alkalies, &c. The above-mentioned salts, when 

 found experimentally to give a brighter color to the blood of the 

 amphibia, are seen to wrinkle the large blood-cells. According to 

 some, oxygen also contracts, and carbonic acid gas expands the cor- 

 puscles ; and herein is another cause of the bright color of arterial, 

 and the darker hue of venous blood. It is also found that if even 

 colorless solid substances strongly reflecting light are mixed among 

 the corpuscles, they give to the mass of blood a brighter red tinge. 



