538 ILLUSTRATIVE EXPERIMENTS 



Another experiment illustrating the effect of electrolytes may be tried. 

 During the tirst tt-10 hours cover the gelatin in the cedometer with a M/8 

 sodium chloride solution. Compare the rate and amount of swelling with 

 that in pure water. Now drain oft' as much of the salt solution as possible and 

 fill up again with pure water. Both rate and amount of swelling are now 

 increased. 



(e) Heat of imbibitio)> . Dry some commercial starch powder at 105° C. and 

 leave to cool in a desiccator. Put about 50 gm. of this into a small beaker, 

 insert a thermometer and read off the temperature. Add about 50 c.c. of 

 water and stir with the thermometer. An increase of temperature of from 

 10-14° C. will be obtained in a few minutes. 



Somewhat better results may be obtained by substituting a vacuum flask 

 for the beaker. 



Potato meal, pease meal, etc., give + 8° to + 10° C. 



(/) EJI^^' ^f various electrolytes on imbibition. First of all, prepare a 

 number of standardised gelatin discs as follows. Make a concentrated 

 solution of gelatin, adding a trace of a colloidal dye {e.g. Congo red) to 

 render the gelatin easily visible. Pour the hot solution upon a glass plate 

 and allow to set. With a large cork-borer (diam. 10-15 cm.), cut into discs 

 which are dried, measured and weighed. Seven Petri dishes are required 

 and are almost filled with the following fluids : (i.) water, (ii.) N/lOHCl or 

 H2SO4, (iii.) N/lONaOH, (iv.) N/2KI, (v.) N/2NH^CNS, (vi.) N/SCaCl.,, 

 (vii.) Sat. MgS04. 



Put a few gelatin discs (not touching one another) into each dish, immersing 

 them quickly and completely to avoid the adherence of air bubbles. After 

 an hour's immersion some of the discs will have visibly swelled. Leave for 

 24 hours and examine against a black background. Measure and weigh. 

 Which disc has swelled most 1 Arrange the electrolytes in a descending 

 order as they have favoured imbibition. This is the so-called hjtropic series. 

 Instead of gelatin, 1 per cent, agar may be used. The lytropic series will be 

 in the same order (see also Experiment 40). 



(g) Effect of Acid on Imbibition, (i.) Stretch a piece of catgut vertically 

 between a weight and a weighted frog-heart lever so that weight and catgut 

 lie in a tall cylinder. The lever may be made to mark a smoked rotating 

 drum. Set the drum going very slowly and almost fill the cylinder with 

 water. Note the changes. Now add sufficient hydrochloric acid to make 

 the whole fluid 1-2 per cent, acid and note the result. 



(ii.) Prepare a series of tubes covering a wide range of acidity and 

 alkalinity, e.g. HCl approximately 5 per cent., 2-5 per cent., 1-25 per cent., 

 0-6 per cent, and 0-2 per cent., and a similar range for NaoCOo. Place a 

 weighed piece of blood fibrin in each and in a tube of water. After half an 

 hour remove the fibrin, dry with blotting paper and weigh. Fibrin absorbs 

 more water in any solution of acid or alkali than in piire water (cf. Experi- 

 ment 42 (c) ). 



(iii.) Remove the eyes from a dead experimental animal or get sheep's eyes 

 [rom the abattoir. Measure the diameters of each eye. Weigh the eyes. 

 Place one eye in one of each of the following solutions, 0-5 per cent. HCl, 

 0-5 per cent. HCl + 3/20 M . Ca(N03)2, 0-5^ per cent. HCl -f N/IO NaCl, 

 0-5 per cent. HCl 4- N/10 Na^SO^. Leave for 6-10 hours and then dry with 

 blotting paper, measure and weigh. The eyes in the pure diluted acid swell 

 very rapidly. The presence of salts prevents much swelling and, in the case 

 of sodium sulphate, may even cause a decrease in weight. Similar results 

 may be obtained with pure gels placed in collodion bags. 



