70 



CARNEGIE INSTITUTION OF WASHINGTON. 



Percentage increase of original thickness, and the water-content per gram of dry weight oj 

 gelatine, at ends of periods indicated and at three percentages of concentration. 



^Dissolving. 



and thickness were placed in water and their increase in thickness 

 measured at intervals. Only sample readings are given in the table. 



Two pieces of gelatine may have had the same water-content when 

 made up and also at the beginning of the test and even then may not 

 absorb at the same rate if their histories between the time of making 

 and the beginning of the absorption were not the same. Suppose a 

 20 per cent gelatine solution is made up, divided into two lots, and 

 allowed to set. Let lot No. 1 be placed in a covered dish, so that no 

 loss of water by evaporation takes place. Let lot No. 2 be allowed 

 (1) to lose water to the air until it has reached either complete or partial 

 air-diyness, and (2) to absorb water until its water-content has returned 

 to the original amount. If now the two pieces are allowed to absorb 

 water simultaneously, No. 2 will absorb at a higher rate than No. 1. 



The previous history of the gelatine used affects also the distribution 

 of the increase in size among the several dimensions of any given piece, 

 as will be seen from the following results obtained from a large number 

 of experiments. The tests were made on gelatine of several concen- 

 trations from 10 to 33 per cent. Rectangular blocks, 2.0 X 0.35 X 

 0.35 cm. were cut from the gelatine as soon as it had set. 



(1) Pieces of the size and shape mentioned above swell equally in 

 thickness, breadth, and length, if no appreciable amount of water has 

 evaporated from the gelatine since it ''set." 



(2) Similar pieces, when placed on a glass plate with the largest 

 face in a horizontal position and allowed to lose water by evaporation 

 at ordinary temperatures, shrink about twice as much in the two short 

 dimensions as in the long one, shrinkage being based on percentage of 

 the original size. 



(3) V/hen the pieces mentioned under paragraph 2 are allowed to 

 absorb water they swell in the same relative proportions that they 

 shrink. That is, the increase in size for the two short dimensions is 

 about twice as great for the long dimension. This distribution of 

 increase continues for at least 63 hours after the gelatine has attained 

 its original water-content. 



