414 SCIENCE PROGRESS 



modifications, one of which is stable above 33° to 35° C, and is 

 denoted as " Sol form A," whilst the other is stable below 1 5° C, 

 and is called " Gel form B." Between 15° and 35° C. a con- 

 dition of equilibrium exists between the two forms, and the 

 mutarotation observed is considered to be due to the trans- 

 formation of one form into the other by a reaction which is 

 reversible with the temperature. In a more recent paper, 

 Davis and Oakes [Journ. Arner. Ckeni. Soc, 1922, 44, 464) 

 have reported that at 38*03° C. gelatin sol and gelatin gel can 

 exist in equilibrium, but that this is not true for any other 

 temperature. For example, a " seeded " solution (one to 

 which a little gelatin had been added) showed no change in 

 viscosity with time at the temperature of 3 8 '03°. At any 

 temperature below this a regular increase in viscosity with 

 time was observed. At higher temperatures a decrease 

 occurred until the viscosity equalled that of a similar unseeded 

 portion at the same temperature. Bogue, however {Journ. 

 Amer. Chem. Soc, 1922, 44, 131 3), has measured the change in 

 viscosity with time of gelatin solutions at varying hydrion 

 concentrations and under different degrees of purity, and 

 found that there are several factors influencing the effective 

 volume, that is, the equilibrium of the gelatin in solution. 

 These include the hydrion concentration (pH), the amount 

 and nature of the inorganic ions present, and the products of 

 gelatin hydrolysis. In another series of experiments Bogue 

 measured the viscosities of gelatin solutions at varying tempera- 

 tures, using a MacMichael viscometer at varying speeds of 

 rotation. He found that if the curves showing the relation 

 between the viscosity and the number of revolutions per 

 minute were extrapolated towards the origin, at temperatures 

 above 34° they met the origin, but below this temperature 

 they intercepted the viscosity axis at distances which were 

 all the greater the lower the temperature. Also, the lower the 

 concentration of the solution, the lower the temperature at 

 which the extrapolated curves still met the origin. These 

 experiments indicate that plastic flow becomes evident in a 

 gelatin solution (25 per cent.) at about 34° C. ; that the tran- 

 sition from the sol to the gel condition is a gradual and not 

 a sudden change, and that in solutions at temperatures 

 below 34° both the sol and gel forms exist together. 



It is obvious from the above that there has been consider- 

 able progress made in the investigation of gelatin, but that 

 there still remains much to be done. As far as the physical 

 properties are concerned, the quantitative investigation seems 

 to have passed through three phases ; in the first phase 

 gelatin was treated mainly as a colloid, in the second mainly 

 as an amphoteric electrolyte, and now, in the third phase, it 



