110 MICROSCOPIC METHODS. 



fluid is allowed to stand. Sedimentation is thus the naked-eye 

 evidence of agglutination. The blood serum may acquire this 

 clumping power towards a particular organism under certain 

 conditions; these being chiefly met with when the individual is 

 suffering from the disease produced by the organism, or has 

 recovered from it, or when a certain degree of immunity has 

 been produced artificially by injections of the organism. The 

 nature of this property will be discussed later. Here we shall 

 only give the technique by which the presence or absence of 

 the property may be tested. There are two chief methods, a 

 microscopic and a naked eye, corresponding to the effects men- 

 tioned above. In both, the essential process is the bringing of 

 the diluted serum into contact with the bacteria uniformly dis- 

 posed in a fluid. In the former this is done on a glass slide, 

 and the result is watched under the microscope; the occurrence 

 of the phenomenon is shown by the aggregation of the bacteria 

 into clumps, and if the organism is motile this change is pre- 

 ceded or accompanied by more or less complete loss of motility. 

 In the latter method the mixture is placed in an upright thin 

 glass tube; sedimentation is shown by the formation within a 

 given time (say 12 or 24 hours) of a somewhat flocculent layer 

 at the bottom, the fluid above being clear. Two points should 

 be attended to; (a) controls should always be made with normal 

 serum, and () the serum to be tested should never be brought 

 in the undiluted condition into contact with the bacteria. The 

 stages of procedure are the following : 



1. Blood is conveniently obtained by pricking the lobe of the ear, which 

 should previously have been washed with a mixture of alcohol and ether and 

 allowed to dry. The blood is drawn up into the bulbous portion of a capillary 

 pipette, such as in Fig. 53, a. (These pipettes can be readily made by draw- 

 ing out quill glass tubing in a flame. It is convenient always to have several 

 ready for use.) The pipette is kept in the upright position, one end being 

 closed. For purposes of transit, break off the bulb at the constriction and 

 seal the ends. After the serum has separated from the coagulum the bulb 

 is broken through near its upper end, and the serum removed by means of 

 another capillary pipette. The serum is then to be diluted. 



2. The serum may be diluted (a) by means of a graduated pipette 

 either a leucocytometer pipette (Fig. 53, b) or some corresponding form. In 

 this way successive dilutions of i-io, 1-20, i-ioo, etc., can be rapidly made. 

 This is the best method. () By means of a capillary pipette with a mark on 

 the tube the serum is drawn up to the mark and then blown out into a glass 

 capsule ; equal quantities of bouillon are successively measured in the same 



