JULY, 1910.] 



7 



Saps and Exudations, 



Grams per 100 c. c. 

 latex. 



Sulphuric acid 01 



Hydrochloric acid 01 



Nitric acid 0'3 



Acetic 0-95 



Oxalic ,, 0-2 



Tartaric 25 



Citric „ ... 05 



Mercuric chloride 0'8 



Formic acid 0'45 



Hydrofluoric acid — 



Acid potassium tartrate ... 016 



In the first place with respect to 

 the acids, an excess has to be avoided, 

 otherwise the coagulation ceases to be 

 complece and rubber globules are left 

 in the mother liquid. The range for 

 complete coagulation is very small with 

 all the acids tested, except acetic. 

 With sulphuric, for example, the amount 

 can hardly be doubled without interfer- 

 ing with the coagulation ; whereas with 

 acetic it can be increased some four 

 times before the residual liquid shows 

 turbidity. In fact, one quarter or nine 

 times the necessary amount of acetic 

 acid may be added with very little 

 waste of rubber — a very considerable 

 range within the limits of which it is 

 easy to keep in practice. For this reason 

 acetic has much the advantage over 

 the others, even though a greater 

 weight is required than in the case of 

 any of the other acids. 



In the second place the quantity of 

 acid needed depends only upon the 

 amount of pure latex, and is independ- 

 ent of its dilution. A definite weight 

 of acid is required to coagulate com- 

 pletely 100 cc of latex, no matter this 

 be diluted to five or ten times its volume 

 with water. The latex of Hevea can 

 . thus be diluted to any extent, and yet 

 its particles of caoutchouc remain cap- 

 able of being drawn together into a clot 

 by the addition of the requisite quantity 

 of acid. This was even done for latex 

 diluted two thousand times. 



The following chain of reasoning was 

 put forward by way of explairing this 

 acid coagulation. " The latex is slightly 

 alkaline. The proteid is of such a nature 

 as to be insoluble in neutral solution, 

 but soluble in alkaline or acid media, 

 i.e., it is an alkali-albumen. When the 

 alkalinity is neutralised by the necessary 

 amount of acid, the proteid comes out 

 of solution and produces with the 

 globules of caoutchouc the clots of rub- 

 ber. If excess of acid be added, then 

 the proteid remains in solution, being 

 now in an acid medium. The acid re- 

 quired for coagulation bears a definite 

 ratio to the quantity of pure latex only, 



no matter what its dilution may be, 

 because the alkalinity is not altered in 

 amount by this dilution. Acetic, being 

 a weaker acid than the others, does not 

 bring about the changes so rapidly." 



The coagulating power cf several salts 

 was tried on the latex of Hevea, None 

 of them ware very satisfactory, except- 

 ing mercuric chloride (corrosive subli- 

 mate). They either gave an incomplete 

 coagulation, or else a large quantity of 

 the reagent was required. A solution of 

 mercuric chloride, however, rapidly 

 separates out the caoutchouc and very 

 little is needed. This is not surprising as 

 the salt is one of the strongest precipi- 

 tants for proteins. Magnesium sulphate 

 was, of the salts tried, the next best 

 coagulant. Mercuric chloride has not 

 been experimented with on a large scale. 

 One serious objection to its use is its 

 very poisonous nature. 



It would, however, be interesting to 

 have a large sample of such rubber 

 reported on by a manufacturer. It 

 furnishes a very tough kind of caou- 

 tchouc, which might possibly have a 

 special use. Such a coagulant has one 

 advantage over acetic acid. It preserves 

 the rubber, whereas that made by 

 means of the acid will mould and rot if 

 kept damp, unless an antiseptic be 

 added. 



Victor Henri's fairly recent researches 

 on the coagulation of latex which had 

 been submitted to dialysis, until no 

 appreciable quantity of saline matter 

 remained in it. Such dialysed latex be- 

 haves differently in regard to coagu- 

 lation. Alcohol, for example, which is a 

 strong coagulant of ordinary latex, has 

 no effect on the dialysed variety, but 

 after the addition of a salt coagulation 

 sets in, hence the presence or absence 

 of salts has much influence on the 

 clotting of latex. 



Henri makes no mention of the albu- 

 minous matter present in the latex, and 

 so it is to be inferred that he regards 

 the coagulating reagents as acting 

 directly on the caoutchouc globules, 

 causing them to agglutinate. But his 

 facts are readily explained otherwise. 

 The dialysis will only remove the salts 

 and not the proteins, is influenced by 

 the presence of salts- When these latter 

 are removed the proteins in solution will 

 behave quite differently towards the 

 various reagents employed for separ= 

 ation ; hence the coagulation of the 

 latex, when dialysed, will not be able 

 to be effected in the same manner as 

 that of the fresh latex ; at the same time 

 the clotting will be due none the less to 

 the protein present. On such reasoning 



