562 ILLUSTRATIVE EXPERIMENTS 



87. Silicic Acid Sol from Water-glass. 



Dilute some " water-glass," "" keep-egg" or other commercial preparation 

 of the same nature with enough freshly boiled (CO2 free) distilled water to 

 reduce its specific gravity to 1-16. This stock solution can be kept for years 

 in a wide-mouthed bottle closed by a rubber bung or a well-vaselined glass 

 stopper. 



To prepare a sol (Hatschek) dilute 30 c.c. of concentrated hydrochloric 

 acid (1-2 specific gravity) with 100 c.c. of water, and pour 75 c.c. of the stock 

 sodium silicate solution info the dilute acid. Dialyse the mixture in a parch- 

 ment bag against repeated changes of water. Caution. Do not push the dia- 

 lysis too far or the sol will set to a gel. 



The sol should be perfectly clear and colourless, and absolutely free of any 

 bluish tinge (which is indicative of impending gelation). The sol may be 

 kept in wide-mouthed bottles as above. It is very sensitive to COo, either free 

 or bound. 



88. Coarse Suspensions. 



Gamboge, Indian Ink, Chinese Ink, etc. A small quantity (1-2 gm.) of the 

 block is placed in a mortar and ground with a few cubic centimetres of w^ater. 

 The resulting mixture is diluted with a large volume (500-1,000 c.c.) of water. 

 The coarser particles are removed by filtration. 



89. (i.) Egg White Solution (from Eggs). 

 [a] Separate white from yolk. 



(6) Beat white to a foam and allow to stand. 



(c) Remove membranous foam from the clear fluid below. 



(d) Add 0-9 per cent, solution of sodium chloride to make the volume 

 five times the volume of (c). 



(e) Add 300 c.c. of water slowly, stirring regularly. 



(ii.) Egg Albumin Sol (from commercial dried egg albumin). Put 10-15 gm. 

 of the commercial albumin ex ovis crushed in a coffee mill or large mortar 

 into 100 c.c. of w^ater in a bottle and leave overnight to allow of imbibition. 

 Add 5 drops of 1 per cent thymol in chloroform and put the bottle on a me- 

 chanical shaker to break up most of the larger lumps. Leave standing a few 

 hours to let the larger lumps settle. Filter through a large glass funnel with 

 a short stem into which has been placed a plug of cotton-w^ool. This filtration 

 will require twelve hours or more, but should yield a yellowish opalescent 

 fluid. 



A more economical but more tedious method is to add the dried egg white 

 to the w^ater in small portions, stirring regularly and attempting to break up 

 the lumps with the glass rod. 



90. Finely Divided Suspension of Protein for Use in Experiments on 

 Proteolytic Enzymes. 



(i.) Take a measured quantity of sheep's blood serum and dilute it with 15 

 volumes of distilled w^ater. Add 10 per cent, sulpho-salicylic acid solution 

 until the mixture imparts a faint violet tint {not blue) to Congo red paper. 

 At this point the diluted serum appears a homogeneous white due to the fine 

 permanent suspension of protein produced. 



(ii.) Milk, centrifuged and filtered from fatty particles through wet paper 

 is a very sensitive substrate for proteolytic enzymes. 



(b) GELS 



91. Egg Albumin. 



(i.) If at {e) in Experiment 89 (i.) the 300 c.c. of water added to the sol 

 were boiling, a fine suspension of coagulated egg white would result. 



