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Tlie Jlj/dration of CoUoi(h. 



By C. LUEDEKING, Ph. D. 

 I. — On th<> vapor leiisioiis and boiling;'- points of Solution.s of l'olIui(Is> 



In a work that Prof. Wiedemann and I published recently "On 

 the heats of hydration of certain Colloids," we determined that 

 the vapor tensions of solutions of gelatine are less than that of 

 water at 40° C. This isolated experiment went only to show that 

 the facts were really as our experiments on heats of hydration 

 would lead us to expect them. When gelatine takes up water it 

 actually becomes chemically hydrated, and holds this water with 

 an affinity whose intensity is indicated by our work. It is, there- 

 fore, held fast by the gelatine so that the vapor tension is less than 

 for pure wattr. Our results led us to doubt the correctness of 

 Guthrie's observations on the boiling-points of solutions of Gum 

 Arabic and of Gelatine, published in Phil. Mag. for Sept., '76' 

 Guthrie says that a 40 p, ct. solution of Gum Arabic boils at 98° 

 C. and that a 45 p. ct. solution of Gelatine boils at 97°-5 C. 

 These results are net in accordance with what we should be led 

 to expect from our thermo-chemical observations. We should 

 expect, as did also Guthrie, that the boiling-points of these solu- 

 tions are higher than that of water. We should expect this also 

 from the fact of their being tenacious liquids. 



It was determined, therefore, to investigate the boiling-points 

 and vapor tensions of Colloid hydrates more fully. Experiments 

 were made on Gelatine, Gum Arabic, Gum Tragacanth, Dex- 

 trine, Starch, and Agar Agar. 



GUM ARABIC 



Pure powdered gum arable was used. A wide test tube con- 

 taining a 40 p. ct. solution was subjected to the heat of a water 

 or brine-bath of 103° C. Fragments of platinum were added to 

 facilitate the formation of vapor. At 90°, when in the water-bath, 

 bubbles formed slowly and regularly, and as the temperature rose 

 their number increased until, at 99°. 8, the temperature to which 



