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BOTAMCAL GAZETTE 



[august 



dries, and adheres. If it does adhere in such manner as to prevent 

 areal shrinkage, it generally is to be removed with difficulty and 

 may be destroyed in the process. 



An improved method of making such plates is briefly as follows. 



i 



First a sheet of gold foil is laid out on a leveled glass plate on a 

 table. Blocks of glass or of some non-corrosive metal are placed 

 on the margins of the gold. The warm agar is poured into this 

 cell in a 2.5 per cent solution, at 40 C, and in cooling to 18 or 

 20 C. it sets, and the blocks may be removed. The gel now 

 stands on a base of gold foil, and unless anchored at the margins 

 will shrink in all dimensions as it dries, a thing which specifically 

 is to be prevented. To do this a little warm solution of agar is 

 run around the agar plates which on cooling cements the margin 

 to the glass. The preparation is now set in a dehydrating cham- 

 ber with a humid atmosphere and subjected to the action of an 

 electric fan for 40 hours. At the end of this time it has come 

 down to a plate about one-fortieth of its original thickness, and 

 may be detached from the plate by cutting away the marginal 

 portion. Properly made, such plates are even as to thickness and 

 swelling qualities. 



When albumin, gelatine, or other albuminous substances are 

 to be mixed with the agar, the solution of the latter is cooled to 

 40 C, and then the other substances in a liquefied condition are 

 poured in and stirred for a few minutes before the plate is poured, 

 which should be done at about 35 C. 



The process puts the experimenter in possession of a sheet of 

 dried material, preferably between 0.15 and 0.20 mm. in thick- 

 ness, and after the margins are trimmed away with the scissors the 

 sheet will probably have a surface of about 7X12 cm. As indicated, 

 these sections may be about 3x5x0.16 mm., with a volume of 

 2.4 cu. mm., and the trio in a dish have a total of 7+cu. mm., into 

 which a measured amount of solution is poured and replaced as 

 the experiment demands. If thinner sections are used, their area 

 should be reduced. 



The auxograph consists essentially of a compound lever set 

 with a vertical swinging arm which rests on the triangular glass 

 plate covering the sections in the dish, and its use in measuring 



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