5. Compress the teat between the finger and thumb (Fig. 193) to such an extent as to drive out the greater part of the contained air. 6. Maintaining the pressure on the teat pass the stem of the pipette into the capsule holding the saline solution, until the open end of the pipette is below the level of the fluid. 7. Now cautiously relax the pressure on the teat and let the fluid enter the pipette and rise in the stem until it reaches the level of the grease pencil mark. As soon as this point is reached, check the movement of the column of fluid by maintaining the pressure on the teat, neither relaxing nor increasing it. 8. Withdraw the point of the pipette clear of the fluid, and again relax the pressure on the teat very slightly. The column of saline solution rises higher in the stem, and a column of air will now enter the pipette and serve as an index to separate the first volume of fluid drawn into the stem from the next succeeding one. 9. Again introduce the end of the pipette into the fluid and draw up a second volume of saline to the level of the grease pencil mark, and follow this with a second air index. 10. In like manner take up seven more equal volumes of saline solution and their following air bubbles. There are now nine equal volumes of normal saline in the pipette. 11. Now pass the point of the pipette into the blood tube and dip the open end below the surface of the serum. Proceeding as before, aspirate a volume of serum into the capillary stem up to the level of the pencil mark. 12. Eject the contents of the pipette into the small tube marked 10 per cent. by compressing the rubber teat between thumb and finger. 13. Mix the one volume of serum with the nine volumes of saline solution very thoroughly by repeatedly drawing up the whole of the fluid into the pipette and driving it out again into the test-tube. 14. Now take a clean pipette and proceed precisely as before, 4 to 10. 15. Having aspirated nine equal volumes of saline into this second pipette, now take up one similar volume of the fluid in the "10 per cent. tube." 16. Eject the contents of this pipette into the second tube marked 1 per cent. and mix thoroughly as before. 17. In similar fashion make the 0.1 per cent. solution and transfer to the third tube. 18. Further dilutions in multiples of ten can be prepared in the same way, and by varying the number of volumes of diluting fluid or serum any required dilution can be made (see Appendix, Dilution Tables). NOTE.--The saline diluting fluid _must always_ be taken into the pipette first, otherwise if the serum contains a very large amount of agglutinin the traces of this serum added to the saline solution may be sufficient to entirely vitiate the subsequent observations--whilst if more than one sample of serum is diluted from the same saline solution serious errors may be introduced into the experiments. ~The Microscopical Reaction:~ _Apparatus Required:_