1920] 



MACDOUGA l—biocolloids 



131 



nor can it be said of the other mucilages or gums already men- 

 tioned as they go into solution from the surfaces of the sections; 

 but sections in which agar forms as much as half may be assumed 

 to return to the original in nearly all work, with a change of not 

 more than 2-5 per cent in area (fig. 1). 



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IS. 



V B 



Fig. r. — Diagrams illustrating action of dried plates of agar and gelatine in 



forming 



thickness 



represented by small inner figure increas 

 and breadth to form larger block of jelly. 



In view of these facts, it is evidently desirable to cast plates 

 and dry sections in such manner that the dried sheet of material 

 will have the areal dimensions of the original, and that ail of its 

 shrinkage will have taken place in such manner as to reduce the 

 thickness only. The preparation of such plates may be done 

 simply by pouring warm solution on a glass plate, where it hardens, 





