1. 25 c.c. require the addition of 5.50 c.c. n/10 NaOH to neutralise. Two controls give the following results: 2. 25 c.c. require the addition of 5.70 c.c. n/10 NaOH to neutralise. 3. 25 c.c. require the addition of 5.60 c.c. n/10 NaOH to neutralise. Averaging these two controls, 25 c.c. require the addition of 5.65 c.c. n/10 NaOH to neutralise, and therefore 1000 c.c. require the addition of 226 c.c. n/10 NaOH, or 22.60 c.c. n/1 NaOH, or 2.26 c.c. n/10 NaOH. Initial _titre_ of the bouillon = +22.6, and as such requires the addition of (22.6 c.c. - 10 c.c.) = 12.6 c.c. of n/1 NaOH per litre to leave its finished reaction +10. But the three titrations, each on 25 c.c. of medium, have reduced the original bulk of bouillon to (1000 - 75 c.c.) = 925 c.c. The amount of n/1 NaOH required to render the reaction of this quantity of medium +10 may be deduced thus: 1000 c.c.:925 c.c.::12.6 c.c.:x. Then x = 11.65 c.c. n/1 NaOH. Whenever possible, however, the required reaction is produced by the addition of dekanormal soda solution, on account of the minute increase it causes in the bulk, and the consequent insignificant disturbance of the percentage composition of the medium. By means of a pipette graduated to 0.01 c.c. it is possible to deliver very small quantities; but if the calculated amount runs into thousandth parts of a cubic centimetre, these are replaced by corresponding quantities of normal or even decinormal soda. In the above example it is necessary to add 11.65 c.c. normal NaOH or its equivalent, 1.165 c.c. dekanormal NaOH. The first being too bulky a quantity, and the second inconveniently small for exact measurement, the total weight of soda is obtained by substituting 1.16 c.c. dekanormal soda solution, and either 0.05 c.c. of normal soda solution or 0.5 c.c. of decinormal soda solution. ~Standardising Nutrient Agar and Gelatine.~--The method of standardising agar and gelatine is precisely similar to that described under bouillon. THE FILTRATION OF MEDIA. ~Fluid media~ are usually filtered through stout Swedish filter paper (occasionally through a porcelain filter candle), and in order to accelerate the rate of filtration the filter paper should be folded in that form which is known as the "physiological filter," not in the ordinary "quadrant" shape, as by this means a large surface is available for filtration and a smaller area in contact with the glass funnel supporting it. To fold the filter proceed thus: 1. Take a circular piece of filter paper and fold it exactly through its centre to form a semicircle (Fig. 100, a). 2. Fold the semicircle exactly in half to form a quadrant; make the crease 2, distinct by running the thumbnail along it, then open the filter out to a semicircle again. 3. Fold each end of the semicircle in to the centre and so form another quadrant; smooth down the two new creases 3 and 3a, thus formed and again open out to a semicircle.