GELATIN 



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This, however, is much better shown in the case of the 

 influence of acids and alkaUs, as is illustrated by the following 

 figures, which are due to von Schroeder. 



Simple swelling experiments may be carried out as follows : 

 A series of gelatin discs of the same size and weight are pre- 

 pared by pouring a concentrated gelatin sol on a glass plate, 

 allowing it to set, and then cutting the set gelatin into discs 

 or squares, and drying. If separate discs are then immersed 

 for twenty-four hours in solutions of N/20-HCI, N/20-NaOH, 

 N/2-KI, N/s-CaClg, water, and saturated magnesium sulphate 

 respectively, it will be noticed that the order in which swelling 

 has been favoured is : acid > alkaU > potassium iodide > 

 calcium chloride > water > magnesium sulphate ; the disc in 

 the acid solution will be swollen to about twice the size of that 

 in pure water. 



In connection wdth swelhng an interesting experiment of 

 Fischer's may be quoted, which throws light on the local swelling 

 which takes place when one is stung by a nettle or a jelly fish. 

 A 6 per cent, gelatin solution is set in a Petri dish, and 

 then infected at various points with formic acid by piercing 

 the gel with a capillary tube containing a concentrated solution 

 of the acid. Water is then poured over the gelatin, and after 

 one or two hours it will be seen that the parts which were 

 infected are more strongly swollen than other parts. 



The results of investigations such as those indicated led 

 to the putting forward of what are known as Hofmeister series, 

 Hofmeister being the one who first studied such effects. For 

 example, the effect of equivalent (tenth-normal) solutions of 

 various acids on the swelling is indicated by the following 

 series : HCl > HNO3 > acetic acid > H2SO4 > boric acid. With 

 the sodium salts of various acids the swelling decreases in 

 the order : Thiocyanates > iodides > bromides > nitrates > 

 chlorates > chlorides > acetates > tartrates > citrates > sul- 

 phates. 



Such series are very difficult to understand, since the order 

 of the compounds does not bear much relation to their ordinary 

 properties ; it is difficult, for example, to comprehend why 

 acetic acid comes between nitric and sulphuric acids. 



It is the realisation that gelatin does not behave simply 

 as a colloid, but also has amphoteric properties, which has 

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