_Diluting Fluids_.--The most widely used are Hayem's and Toisson's. Both of these have high specific gravities, so that, when well mixed, the corpuscles do not separate quickly. Toisson's fluid is probably the better for beginners, because it is colored and can be easily seen as it is drawn into the pipet. It stains the nuclei of leukocytes blue, but this is no real advantage. It must be filtered frequently. HAYEM'S FLUID. TOISSON'S FLUID. Mercuric chlorid 0.5 Methyl-violet, 5 B 0.025 Sodium sulphate 5.0 Sodium chlorid 1.000 Sodium chlorid 1.0 Sodium sulphate 8.000 Distilled water 200.0 Glycerin 30.000 Distilled water 160.000 {155} _Sources of Error_.--The most common sources of error in making a blood count are: (_a_) Inaccurate dilution, either from faulty technic or inaccurately graduated pipets. The instruments made by Zeiss can be relied upon. (_b_) Too slow manipulation, allowing a little of the blood to coagulate and remain in the capillary portion of the pipet. (_c_) Inaccuracy in depth of counting chamber, which sometimes results from softening of the cement by alcohol or heat. The slide should not be cleaned with alcohol nor left to lie in the warm sunshine. (_d_) Uneven distribution of the corpuscles. This results when the blood is not thoroughly mixed with the diluting fluid, or when the cover-glass is not applied soon enough after the drop is placed upon the disc. _Cleaning the Instrument_.--The instrument should be cleaned immediately after using, and the counting chamber and cover must be cleaned again just before use. Draw through the pipet, successively, water, alcohol, ether, and air. This can be done with the mouth, but it is much better to use a rubber bulb. When the mouth is used, the moisture of the breath will condense upon the interior of the pipet unless the fluids be shaken and not blown out. If blood has coagulated in the pipet--which happens when the work is done too slowly--dislodge the clot with a horse-hair, and clean with strong sulphuric acid, or let the pipet stand over night in a test-tube of the acid. Even if the pipet does not become clogged, it should be occasionally cleaned in this way. * * * * * III. COLOR INDEX This is an expression which indicates the amount of hemoglobin in each red corpuscle compared with the normal amount. For example, a color index of 1.0 indicates that each corpuscle contains the normal amount {156} of hemoglobin; of 0.5, that each contains one-half the normal. The color index is most significant in chlorosis and pernicious anemia. In the former it is usually much decreased; in the latter, generally much increased. In symptomatic anemia it is generally moderately diminished. * * * * *