Theoretically, therefore, the amount of citrate that should be used as an anticoagulant should be 2 grammes for 1,000 cc. of blood, or 100 cc. of 2 per cent. solution for 900 cc. of blood. In practice it is better to err on the side of safety and to use a slight excess of citrate. This amount of citrate should be used for the 750 cc. of blood which constitutes the ordinary maximum amount of blood used in a transfusion. For smaller quantities of blood the amount of citrate may be correspondingly reduced. The use of citrated blood was introduced to the British Army in France in 1917 by Oswald Robertson, who recommended the use of a larger amount of citrate than this. His object in increasing the amount was to produce a solution which, when diluted with the correct amount of blood, would be isotonic with it. It was thought that a hypotonic solution might result in some damage to the red corpuscles by osmosis, and Robertson therefore recommended the use of 160 cc. of a 3·8 per cent. solution of citrate, which, when mixed with 750 cc. of blood, will give a solution of which the osmotic pressure equals that of 0·9 per cent. saline solution. It may be doubted, however, whether this consideration is of more than theoretical importance. There can be little doubt that in practice the effect of a slightly hypotonic solution, such as is given by the 100 cc. of 2 per cent. solution of citrate, is negligible as regards destruction of corpuscles. If, however, it be thought necessary, an isotonic solution may be produced by the addition of sodium chloride. Other considerations, as will be seen shortly, weigh in favour of giving the smaller amount of citrate. The dosage to be recommended, therefore, on practical and experimental grounds is 2 grammes of citrate in 100 cc. of water for 900 cc. of blood, or 1 gramme of citrate in 50 cc. of water for 450 cc. of blood or less. These proportions need not be observed very accurately. Latitude may be used in either direction without harming either the transfused blood or the patient.