_Diluting Fluids_.--The diluting fluid should dissolve the red corpuscles so that they will not obscure the leukocytes. The simplest fluid is a 0.5 per cent. solution of acetic acid. More satisfactory is the following: glacial acetic acid, 1 c.c.; 1 per cent. aqueous solution of gentian-violet, 1 c.c.; distilled water, 100 c.c. These solutions must be filtered frequently. * * * * * V. ENUMERATION OF BLOOD-PLAQUES The average normal number of plaques is variously given as 200,000 to 700,000 per c.mm. of blood. The latter figure probably more nearly represents the true normal average, since the lower counts were obtained for the most part by workers who used unreliable methods. Physiologic variations are marked; thus, the number {166} increases as one ascends to a higher altitude, and is higher in winter than in summer. There are unexplained variations from day to day; hence a single abnormal count should not be taken to indicate a pathologic condition. Pathologic variations are often very great. Owing to lack of knowledge as to the origin of the platelets and to the earlier imperfect methods of counting, the clinical significance of these variations is uncertain. The following facts seem, however, to be established: (_a_) In acute infectious diseases the number is subnormal or normal. If the fever ends by crisis, the crisis is accompanied by a rapid and striking increase. (_b_) In secondary anemia plaques are generally increased, although sometimes decreased. In pernicious anemia they are always greatly diminished, and an increase should exclude the diagnosis of this disease. (_c_) They are decreased in chronic lymphatic leukemia, and greatly increased in the myelogenous form. (_d_) In purpura hæmorrhagica the number is enormously diminished. Blood-plaques are difficult to count owing to the rapidity with which they disintegrate and to their great tendency to adhere to any foreign body and to each other. The method of Kemp, Calhoun, and Harris is practical and is to be recommended: * * * * *