CHAPTER III 



THE PREPARATION OF THE JELLY FILM 



AGAR, the substance used for making the film on 

 which the cells are examined, is obtained from sea- 

 weed. It is very cheap, and may be bought in strips 

 or as a powder. We have used Merck's powdered 

 agar, which is quite neutral and pure. It is insoluble 

 in cold water, but immediately soluble in boiling water. 

 This solution, therefore, on cooling, sets as a jelly. 

 It is necessary to have a stock of jelly constantly in 

 hand, and a 2-per-cent preparation is used throughout. 

 This will melt when its boiling-point is approached, 

 but will not set again until the temperature has fallen 

 almost to 40 C. Unlike gelatine, this jelly may be 

 boiled over and over again, and it will always set at 

 its usual temperature. 



The jelly is made in 2-per-cent strength for the 

 reason that it will stand diluting with its own volume 

 of water or other solution, and will still set as a 

 jelly that is, a 1-per-cent solution of agar jelly will 

 set on a slide in the form of a film as it cools. By 

 using the 2-per-cent preparation we are enabled 

 to add an equal volume of any solution we please, 



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