Abstract
Auxographic measurements of the swelling of sections of dried plates of agar and of gelatine previously described show that the relative enlargement of a colloidal body in its different axes will be determined largely by the unequal stresses which may be set up, as for example when liquid agar or gelatine is poured on glass and dried without shrinking in area. It was pointed out that sections from such plates of agar increased only 3 or 4 per cent in length while swelling 3,000 or even 4,000 per cent in thickness, and that sections of gelatine increased 8 to 40 per cent in length while swelling from 500 to 2,000 per cent under the auxograph. 1
Tests of sections of plates of pure agar freshly made and a year old have recently been made. Plates which swelled 2,000 per cent in water when freshly made August 1, 1919, increased but 1,600 per cent July 1, 1920. Plates swelling 3,200 per cent when young increased but 2,000 per cent when nine months old. This total decrease was accompanied by lessened swelling in thickness and increased swelling parallel to the broad surfaces of the plates. The relative increase in length and width of sections of old plates was double that in the same plates when newly made and swelled in water. Similar increases occurred when old plates were hydrated in chlorides of K, Na, Mg and Ca at 0.0001 M.
The effects of age on gelatine plates are not so marked but the areal stlrelling increases with age. The differential effects of the various solutions on such areal or linear increases were very marked and noticeable. Thus strips 30 to 50 mm. in length cut from a single plate when placed in the solutions gave increases in thickness and length as below:
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