CARBOHYDRATES 137 



ology. Finally, we must mention the pectinous plan-t juices, which 

 unlike the true gums are slightly or not at all soluble in water. 



The gums are typical examples of hydrophile colloids; they swell 

 into jellies in water, and on adding more water pass, at an indefinite 

 point, into solution. Rise of temperature shifts this point in favor of 

 solution, though it is by no means immaterial at what condition of 

 swelling the heating occurs. If, for instance, agar has been allowed to 

 swell in cold water for a long time, it immediately becomes a homo- 

 geneous solution on warming. If solid agar is heated in water, we 

 get a lumpy suspension of agar in water, which only very gradually 

 becomes a homogeneous sol. It is evidently necessary for each 

 particle of agar to have the amount of water necessary for solution 

 in close proximity before it is wanned; otherwise the swelling will 

 occur but slowly from the outside, where there is an excess of water, 

 and proceed inward, since the peripheral particles of agar hold the 

 water until they are dissolved. Indeed, the phenomenon is one which 

 depends on the size of the surface; the large mass with relatively 

 small surface dissolves more slowly than the same mass divided, 

 i.e., with a relatively increased surface. Solutions of gum do not 

 dialyze. In my opinion little attention need be paid to the determi- 

 nation of their osmotic pressure, since traces of electrolytes which 

 cannot be removed, suffice to simulate it. I know of no studies 

 on the electrical migration or on the diffusion coefficients of gums. 

 [W. M. BAYLISS has recently determined the viscosity and osmotic 

 pressure against water and RINGERS' solution of gum acacia, gelatin 

 and amylopectin. He recommends the use of gum and gelatin in 

 saline infusions as a method of maintaining blood pressure. The 

 more prolonged action of such infusions he attributes to the osmotic 

 pressure of the colloids. Proceedings of the Royal Society of London, 

 Series B, No. 89, pp. 380-393. Tr.] 



Gums usually diminish the surface tension of water. The a- of a 

 20 per cent solution of gum arabic is 9 per cent lower, and a dilute 

 solution of agar 5 per cent lower than that of water (G. QUINCKE). 

 Some kinds of gum increase the surface tension of water (ZLOBICKI*). 



The general facts, stated on page 66, hold for the swelling and 

 shrinking of gums. On swelling, the heat liberated, according to 

 E. WIEDEMANN and CHAS. LUDEKING,* is 9.0 cal. per gin. for gum 

 arabic and 10.3 cal. per gm. for gum tragacanth. Wo. PAULi* 1 found 

 that a rise of temperature accompanied the swelling of carraghen. 



The significance of crystalloids for swelling and turgor has been 

 studied chiefly in gelatin. In the case of the gums, other than agar, 

 no investigations of this point have been made. Though the prob- 

 ability of many similarities exists, an absolute parallelism cannot be 



