Heidenhain recognizes two classes, first, such substances as peptone, leech extract and crayfish extract; and, secondly, crystalloids such as sugar, salt, &c. Starling sees no reason to believe that members of either class act otherwise than by increasing the pressure in the capillaries or by injuring the endothelial wall.
Much more important is the effect of the alteration in the amount of crystalloids in the tissues and blood and therefore of the alteration in the osmotic pressure between these.
Now differences in the amount of crystalloids cause alteration in osmotic pressure while the proteid content affects it but little; and of the crystalloids the chlorides appear to be those most liable to variation.
As sodium chloride is one of the most permeable of crystalloids it seems strange that damage to the renal tissue should impede its excretion.
Some Siphonales (Codium) give rise to proteid crystalloids, and they are of constant occurrence among Florideae.
His early work on the movements of gases led him to examine the spontaneous movements of liquids, and as a result of the experiments he divided bodies into two classes - crystalloids, such as common salt, and colloids, of which gum-arabic is a type - the former having high and the latter low diffusibility.