STARCH, SUGAR, ETC.] 



CHEMISTRY. 



411 



These illustrations will be sufficient to give our 

 readers a general idea of the calculations involved in 

 organic analysis, with the exception of that of the per- 

 centage, which is precisely similar to that explained 



already.* We shall now proceed to investigate the pro- 

 pertiesof numerous organic substances commencing with 

 starch, sugar, <tc. ; and subsequently shall examine their 

 chemical products, which are of a more complex character. 



CHAPTER VII. 

 STARCH, SUGAR, ETC., AND THEIR PRODUCTS. 



Iy commencing the chemical history of vegetable pro- 

 ducts, we may observe th;it such must be to a large 

 extent very cursory. The advance which chemical 

 science hag made during the last fifteen years in respect 

 to the branch of organic products, has been so extensive, 

 that were we to attempt to enter into it fully, we should 

 require mo*e space than lias been devoted to all the sub- 

 jects yet treated on iu this volume. We are, therefore, 

 compelled to select and indicate, rather than to define, 

 all the divisions of this interesting department of ex- 

 perimental science. 



The first substance to which we shall direct the atten- 

 tion of our readers is starch in its various forms. Starch 

 is to be found in almost every plant, in some form or 

 r. The most familiar instances which we can adduce 

 of iu presence, is that of its being found in wheat, 

 potatoes, and other such plants as are used for common 

 articles of food. If flour be carefully washed with cold 

 water, a milky liquid passes off, which contains the 

 starch of the wheat. If the liquid be allowed to settle, 

 the starch will be deposited as a white powder, being 

 separated from the gluten, which is the other component 

 of the flour. If potatoes are broken up and pounded, 

 the starch of which they are chiefly composed may be 

 separated in a similar manner. During the existence of 

 the potato disease, the fact that much starch may be 

 obtained from potatoes, even in a diseased state, was 

 utilised ; and large quantities of that substance were 

 thus obtained for the use of calico-printers, by which a 

 considerable amount of wheat, previously used for that 

 purpose, was saved. Starch forms, also, the chief part 

 of arrow-root, sago, tapioca, and most other farinaceous 

 foods. 



The physical characters of this substance vary with 

 the sources whence it is obtained. To all external ap- 

 pearance, as viewed by the naked eye, it seems merely 

 a granular powder ; but if placed under a microscope, 

 the various kinds of starch show cells, differing consider- 

 ably in their external characteristics. The following 

 engraving and description will assist our readers in this 

 respect ; and if they possess a microscope of ordinary 

 power, they may readily make their own examinations 

 of various forms of feculine products. 



If starch be mixed with cold water, no notable change 

 takes place ; on mixing it with boiling water, however, 

 the cells swell enormously, and eventually burst, as is 

 familiarly known to be the case in cooking potatoes, 

 making paste, and starch, for laundry use. By these 

 means the real starch, which is enclosed in a membrane, 

 is set free by the heat rupturing its covering. 



We shall have eventually to notice the office which 

 starch fulfils in reference to the propagation and growth 

 of plants, and the simultaneous production of sugar. 

 We shall now proceed to describe its chemical characters. 



Starch is composed of carbon, hydrogen, and oxygen. 

 Its symbol and proportions are C 12 H, O, . It is 

 precipitated by metallic oxides, and other substances in 

 solution. Its proper test, however, is iodine, with 

 which it forms a dark-blue colour, provided a cold 

 solution of the test be employed. The simplest way of 

 detecting its presence, is to add a solution of iodide of 

 potassium to the substance supposed to contain starch ; 

 a few drops of sulphuric acid are then to be poured into 

 the mixture ; and if starch be present, a dark indigo 

 colour is at once produced. A heated solution affonls 

 no such reaction. Starched paper dipped in a solution 

 of iodide of potassium, is a test for the presence of free 



ozone, which sets the iodine free from the salt, and so 

 produces a blue colour in proportion to its presence in a 

 free state. 



Fig. 83. The more common fornu of the Starch Cell. 



a, rice, Terr imIl and angular ; , airn, larger, rounded, and granular ; 

 c. wheat, larger, rounded, and granular, faintly marked with concentric 

 - ; rf. potato, medium fi/c. flattened, and with well-marked lines ; 

 f. the tame, more higblr magnified, BO aa to show the nucleus 1, and 

 the markings. 2 ; /. I'out let Muii, the largest kind of starch, of oval 

 shape, well-developed markings, and sonu-times with a double hilnni, 

 I ; g, the same, ruptured br the application of heat. BO that the mem- 

 brane at A is retracted and corrugated, and the contents exposed ; t, 

 the starch of the common pea (Pi*>im} t with iu deep crntral folding or 

 cavity. The precise figure of this cavity or folding, differs in various 

 grains. 



By boiling starch in dilute sulphuric acid, a substance 

 called DBXTKIN is produced, which derives its name 

 from the fact that it turns a ray of circularly polarised 

 light towards the right hand. LIONIN is a substance 

 forming a large proportion of cotton, and other fibrous 

 materials ; and these, by being boiled with diluted sul- 

 phuric acid, may be readily converted, by the change of 

 their ligniu, into grape-sugar. Of this singular fact we 

 shall have to speak more fully hereafter. 



GUMS proper, soluble in water, are a common pro- 

 duct of plants. Soluble in cold water, they afford an 

 adhesive solution, which is used for a variety of pur- 

 poses. The ordinary gum-arabic may be taken as a 

 type of the class ; and from it a substance called arabin 

 may be obtained, which is composed of C 12 H,! Ojj ; 

 and thus has a similar composition to sugar. The pre- 

 sence of gums in solution, is shown by their precipitation 

 with the neutral or basic acetate of lead, also in solution. 



INCIIN is a substance analogous to starch, from which, 

 however, it differs, in giving a brown colour when tested 

 by iodine. It is insoluble in cold water, but dissolves 

 in that liquid when heated. 



Purr IN is somewhat analogous to starch. It is the 

 jelly of fruits, such as is produced in the culinary pro- 

 See ante, p. 405. 



