206 



vegetable tissues, and with small quantities all efforts are in vain ; 

 with large quantities the final result depends on the vegetable 

 tissues being reduced to a proper state by bruising, cutting, &c., 

 and even then a small percentage of starch is retained tenaciously 

 by the cellular membranes. Dry substances must be pulverised 

 as fine as possible, and are converted into a paste with water; 

 fleshy or tough parts have to be treated on a g-rater; the mass, ob- 

 tained in either way, is brought on a square or circular piece of 

 silk gauze (so-called bolting silk-gauze, Nos. 10-13), the latter is 

 made to assume the shape of a bag and tied so as to enclose the 

 contents firmly. The whole bag is then put into a basin contain- 

 ing pure water, is held by one hand above the knot, and is 

 kneaded by the forefinger and thumb of the other hand. After 

 the water has become very milky, it is poured into a glass 

 jar capable of holding at least four to six times more water. 

 Now put the bag into fresh water and knead again for some time, 

 pour the liquid into the glass, and repeat these operations as often 

 as the water becomes milky. Afterwards allow the liquids, all 

 mixed together in the glass jar, to subside, collect the sedi- 

 ment on a filter, wash with pure cold water until the filtrate is 

 found to leave no residue on evai)orating, dry at first at a 

 temperature not exceeding 40°, afterwards at 100^, and determine 

 the weight. 



Usually, instead of silk-gauze, linen or calico is used for the 

 same purpose, but the starch-grains cannot pass through linen 

 with equal facility; more kneading is therefore required, causing 

 other parts (fibres, membranes) to pervade the pores and to con- 

 taminate the product. Even with silk-gauze part of those im- 

 ])urities are lialjle to pass through, unless another covering of the 

 bag of the same material be employed, in which case it is possible 

 to obtain the starch as nearly pure as possible. 



After the Starch has been obtained, the remnant has to be 

 tested on its thorough exhaustion by throwing a small sample, 

 Avhile moist, into a porcelain dish containing a solution of iodine 

 diluted to a gold-yellow colour. If no violet or blue colouration 

 ensues, the remnant is free from Starch; but if the latter be indi- 

 cated by a more or less blue tinge, its quantity is determined in 

 another way, suitable also in all cases where the mechanical pro- 

 cess would be impracticable. But before this can be done, the 

 substance has to be exhausted first successively by ether, alcohol 

 and cold water in order to free it from any traces of sugar, gum, 

 &c., which might be present. 



This indirect estimation of Starch is effected by converting it 

 into »rape-sugar and by submitting the latter to the agency of an 

 alkaline solution of sulphate of copper, containing tartaric acid 

 (see under "Reagents," in Part II.). For this purpose the 

 .suljstance is dried after having been treated to the above-named 



