142 ORIGIN AND NATURE OF LIFE 



and diagnosing various kinds of cells, and in 

 recognizing under the microscope the micro- 

 organisms which produce disease. 



A division of colloids has been mentioned 

 above into reversible and irreversible, but it 

 is probable that even irreversible colloids, 

 if treated delicately enough, can be reversed 

 if the process is not carried too far. This has 

 been shown by the aid of the ultra-microscope, 

 using one of the protein bodies of the blood 

 plasma. If a solution of this colloid placed 

 in the field of the ultra-microscope, be 

 cautiously warmed to just below the point 

 of temperature which would cause it to 

 coagulate irreversibly, then it is observed 

 that the view which was previously clear 

 becomes suddenly filled with an innumerable 

 host of moving particles which rapidly 

 increase and become more prominent. If 

 at this stage the heating process be stopped 

 and the solution allowed to cool slowly, as 

 the temperature falls all these particles 

 dissolve and disappear, and this appearance 

 and disappearance can be many times repeated. 

 If the heating be carried so far, however, 

 that particles visible microscopically even to a 

 high power of the ordinary microscope are 

 formed, then the process ceases to be 

 reversible. 



