16 



Hydration and Growth. 



The most useful amount of material for making dried plates of a 

 thickness of 0.3 mm. or less is that which includes 10 grams of dry 

 material made up to a 2 per cent liquid or solution. This amount will 

 form two plates about 8 by 15 cm. which will come down to the 

 desired thickness when dried at 15° to 20° C. Two methods of casting 

 such dried plates may be used, according to the composition of the 

 colloid that is being manipulated. Gelatine, or plant mucilages, or 

 mixtures in which these substances compose half or more of the whole, 

 must be poured directly on glass plates or on sheets of gold or plati- 

 num foil. 



A sheet of plate-glass with a good surface is set in a level position 

 after the surface has been well cleaned and polished. A cell about 

 8 mm. in depth and 10 by 15 cm. is now made on it by glass strips fitted 

 together so closely than when the warm material is poured onto the 

 glass it will not leak out at joints or corners. After the mixture has 

 cooled sufficiently for the gel to set, the plate is placed in the desiccator 

 and drying should be carried on at such rate that no further loss of 

 weight occurs after about 40 hours. The dried plate is now worked 

 free from the glass at one margin by an instrument with a chisel edge 

 and then stripped free, after which it should be placed in a closed glass 

 dish to keep it free from dust and undue desiccation, which would 

 produce buckhng or warping. 



Fig. 2. 

 Drying frame. A sheet 

 of wire gauze of 1 mm. 

 mesh stretched on a 

 heavy wooden frame, 

 being fastened secure- 

 ly in place by a tightly 

 fitting strip of wood 

 which carries the mar- 

 gin of the netting down 

 into a groove in the 

 frame, as shown in the 

 smaller detail drawing. 

 A, molding form of 

 brass bars; B, margin 

 of wire netting and 

 clip fastening it in 

 place; C, wire netting 

 as it clears frame. The 

 wire netting and brass 

 bars are coated with 

 fine shellac. 



Many plates are ruined in the method described, and when possible 

 the following devices will be found useful: A sheet of rustless wire 

 screen with a mesh less than 2 nam. is stretched on a heavy wooden 

 frame (see fig. 2), so as to offer a good plane surface. A mold of four 

 brass bars is laid on the surface and a sheet of hard filter-paper, such 

 as Whatman No. 40, is fitted into this cell. The frame is placed in a 

 level position and the mixture poured into the shallow cell to a depth 

 of about 8 mm. (fig. 3) . After it has cooled and set, the brass members 



