'2'2'2 Frederick Guthrie on Salt Solutions 



— 0°"2 C. A 10-per-cent. solution separates ice at 0° C, and 



forms a solid mass at — 0°*3 (J. The boiling-points of albu- 

 minoid solutions, of course, could not be determined. And 

 the addition of alkalies to prevent coagulation would naturally 

 vitiate the results. 



§ 1(34. Gelatine. — The gelatine used in the following ex- 

 periments was such as is commonly used by confectioners. 

 Solutions of this substance containing 5, 10, 15 and 20 per 

 cent, may be rapidly cooled under constant stirring without 

 gelatinization. Ice in all such cases begins to be formed at 

 0° C. ; and the whole becomes solid at that temperature. 

 Stronger solutions are so difficult to manipulate, on account 

 of their gelatinization and consequent hardness, that the re- 

 sults with them are doubtful. As far as the experiments 

 extend, however, they fail to show that the freezing-tempera- 

 ture is affected. 



A 20-per-cent. solution heated (as in § 162) in a brine-bath 

 boiled at 99°-9 C. A 45-per-cent. solution boiled at 97°-5 ; 

 and a 50-per-cent. solution, which is as stiff as the strongest 

 glue used by cabinet makers, boiled also at 97°*5. A very 

 striking and conclusive experiment is, to surround the tube 

 containing the 50-per-cent. solution with distilled water in 

 another tube. If the exterior water-tube be heated slowly, 

 the glue in the inner tube boils before the water in the outer 

 one. On continually stirring the frothy viscid mass, the true 

 boiling-point is observed to be constant at 97°*5. 



As an experiment the above is exceedingly instructive, and, 

 I believe, novel ; but, like so many novel and instructive pheno- 

 mena, it is one of daily occurrence. The blistering and froth- 

 ing of the strongest glue when heated by steam or water in 

 the outer pot has certainly been observed hundreds of times 

 in every carpenter's shop. Is it strange or is it natural that 

 so familiar a fact should never have struck us as requi ring- 

 explanation ? 



§ 165. Mixtures of organic colloids. — To round off the series 

 of experiments with colloids homologous with those of crys- 

 talloids, I next examined a mixture of gum arabic and gela- 

 tine. A solution was made containing 20 per cent, of gela- 

 tine, 20 of gum arabic, and 60 of water. This began to freeze 

 and froze completely at 0° C. It boiled at 97°-7 C. 



(4) General remarks on colloids. 



§ 166. The depression of the boiling-point of water by the 

 solution in it of organic colloids has its analogies. Amongst the 

 metals innumerable instances occur to us in which the melting- 

 point of an alloy is not only lower than that of one constituent, 



