394 THE BLOOD, BY ALEXANDER ROLLETT. 



blood by mechanical means, in which case it is only possible to 

 observe the final changes effected in the corpuscles by the re- 

 agent under the microscope ; or the plasma or serum of the 

 blood corpuscle may be washed away with the reagent, under the 

 microscope, in the manner described at p. xx. of the introduc- 

 tion to this work, in which case, in order to prevent the cor- 

 puscles from floating off, it will be found advantageous to spread 

 upon the slide a thin layer of a felt-like mass of fine clean 

 asbestos, or of scraped Swedish filtering paper, and to place 

 the blood drop on this ; or, lastly, the blood and the reagent 

 may be placed in close proximity with each other, and allowed 

 to diffuse slowly. 



It is only when, in the process of washing by the first 

 method, the several blood corpuscles exhibit differences in their 

 behaviour with the reagent that we are justified in concluding 

 that an internal and original difference exists between them. 



It is not permissible to draw this conclusion when the second 

 and third methods are employed, or at least only providing 

 that very great caution has been exercised; for if the uni- 

 formity of the mixture has not been constantly maintained, 

 some of the corpuscles will necessarily be first and more ener- 

 getically acted on by the reagent, and the amount of change in 

 any instance will be proportionate to the duration of the ex- 

 posure to its influence. We may very easily satisfy ourselves 

 that the changes effected by one and the same reagent are very 

 different during the first period of its action, and lead to other 

 results than at later periods. 



The many difficulties that encompass the study of the opera- 

 tion of reagents on the blood have not, as a rule, received 

 sufficient attention ; and less, perhaps, has been accomplished 

 by this mode of experiment than might otherwise have been 

 the case. 



a. The addition of water renders the surface of the cor- 

 puscles smooth, and so changes their various diameters that 

 they become spherical,* and thus acquire that form which with 

 a given surface can contain the largest amount of material. 

 This effect is commonly indicated as a process of imbibition, a 



* Hewson, Opus posthumum, p. 25. 



