July 27, 1883.] 



♦ KNOWLEDGE ♦ 



53 



gum." Here, then, are three bodies, in many respects 

 widely different from each other, and yet apparently alike 

 from a chemical point of view. Supposing they were sub- 

 jected to analysis, the result in each case would be carbon 

 44-45, hydrogen G17, and oxygen 49-38 per cent. ; from 

 these results the chemist would for each calculate as 

 the simplest formula that just given above. In many cases 

 he can further state with certainty that the molecule or 

 ultimate group of atoms of each compound contains only 

 the number of atoms represented by this simple formula ; 

 but in other instances he finds with equal certainty that the 

 formula must be doubled. To make this clear by an 

 example, the calculated formula of oxalic acid is HCO„ ; 

 other considerations, however, leave no doubt that the 

 molecule actually contains H^C^O^. We are here pre- 

 suming that it is self-evident that, whichever be the 

 formula, the percentage composition would be identical. It 

 is not so easy to say with starch and these other bodies 

 whether their formuhe are multiples or not of that above 

 given. It is very probable that they are such multiples. 



This much we know, that there are not only widely 

 different chemical compounds whose formulae only differ by 

 being multiples of each other, but there are also others 

 whose formuhe are actually identical, notwithstanding the 

 very opposite characters they may present. In endeavour- 

 ing to trace out the cause of this we must look to some 

 different arrangement of the atoms within the molecule. 

 The chemical elements and the chemical compounds may be 

 aptly compared to the letters and words of a language ; just 

 as out of twenty-six letters, millions of word combinations 

 are possible, so with our few chemical elements an infinity 

 of chemical compounds may be produced. We know, 

 further, that out of the same letters, differently arranged, 

 we may form several words ; thus the letters a, r, t give us 

 respectively the words, art, rat, and tar ; so in a similar 

 manner may we suppose that ditt'erent groupings of the 

 atoms within a molecule may yield a variety of chemical 

 compounds. Modern chemical research has gone a step 

 further than this, and in many instances tells us exactly 

 how the molecules are grouped together in the compound, 

 and what the difference in grouping is that causes two 

 bodies of similar percentage composition to vary so much 

 in character. 



Cellulose is white, tasteless, and insoluble in water ; it 

 may, however, be dissolved by a solution of copper oxide 

 in ammonia, and can again be precipitated by tlie addition 

 of hydrochloric acid. Starch and cellulose may be readily 

 distinguished from each other by the addition of iodine ; 

 the latter compound remains colourless, or, at most, is 

 stained slightly yellow. Starch, on the contrary, becomes 

 an intense blue, so dark with much iodine as to appear 

 black. This test is a most important one for starch, and 

 may frequently be used in every day life for its recognition. 

 It may be employed in the following manner : — Make a 

 dilute solution of iodine in methylated spirits, or dilute 

 the pharmaceutical tincture of iodine with spirits, until of 

 a deep sherry tint ; the re-agent is now ready for use. 

 Make a solution of starch and add the iodine solution, 

 notice the intense blue colour produced ; on boiling the 

 liquid the colour vanishes, but reappears on cooling. This 

 test may be used for determining which are the starch-cells 

 in microscopic sections of wood, A-c, for on moistening the 

 specimen with a very dilute iodine solution, the starch 

 granules become blue. The uses of cellulose in the forms 

 of cotton, lin(!n, and paper are familiar to all. 



Let us in the next place see what relation starch and 

 cellulose bear to the gummy matter of the cereals. In our 

 series of analyses, quoted in the former article, gum and 

 sugar are classed together. The amount present varies 



from 1-6 in rice to 11-3 per cent, in rye ; wheat is inter- 

 mediate, with a percentage of oS. It has been already 

 stated that this particular form of gum also has the formula 

 CuH50,o. From its powerful action on polarised light, it 

 has received the name of " dextrin." The optical beha\"iour 

 of dextrin and its allied bodies will furnish ample subject- 

 matter for a separate article of the series. Starch and 

 cellulose, if subjected to heat, become altered in character. 

 This is more particularly seen with starcL By heating 

 starch or fiour to a temperature of about 150" C, it is found 

 to undergo a slow change ; it darkens in colour and 

 acquires the odour of highly-baked bread. On now treating 

 with water, a substance is dissolved out, which possesses 

 all the most important properties of gum arable ; on evapo- 

 ration, we have dextrin remaining as an uncrystallisable, 

 solid, and translucent body. Dextrin is applied to a 

 variety of useful purposes, among others the stiti'ening of 

 calicoes, in which instance it is used as a substitute for its 

 allied compound, cotton cellulose. Xotwithstanding the 

 intimate relationship of these two bodies, the substitution 

 of dextrin is not altogether to be recommended, for although 

 the material may look well in the hands of the shopman, 

 the soluble dextrin, after the first washing, manages to avoid 

 taking its share of the fair wear and tear of the article. 

 Dextrin is also used for producing the adhesive layer on 

 the back of postage-stamps. It is said to have been 

 first discovered at the burning of a starch manufactory. 

 One of the firemen, worn out by the exertions of ex- 

 tinguishing the fire, lay down in his garments, wetted by 

 the water that had drained from the heated starch, and 

 awoke to find himself firmly glued down to his resting- 

 place. The production of dextrin, from starch by heat, 

 was thus first noticed, and gave rise to its further manu- 

 facture. Should this account be the true one, many readers 

 may see an analogy to the Chinese discovery of roast pork, 

 by the accidental burning of a pig-stye : while rumour has 

 it that the Chinese for some time burnt a pig-stye when- 

 ever they wanted roast pork — it is not the custom of 

 English manufacturers to burn starch factories for the 

 purpose of maintaining the supply of dextrin. [Charles 

 Lamb, in his famous essay on Roast Pork, says a housf 

 was burnt to supply roast pork in the early stages of men's 

 faith in that edible. I may note also that the account 

 given in Dublin of the origin of the manufacture of the 

 postal gum differs considerably from that given above. A 

 whiskey store, not a starch manufactory, appears, and the 

 national potato (introduced, as is well known, by St. 

 Patrick, when he came over from his native city Glasgow 

 for the purpose of driving out snakes from Ireland) plays 

 an important part in the story. — R. P.] 



THE GREAT PYRAMID. 



MISS A. B. EDWARDS AXD ilR. K. A. PKOCTOR. 



[I have received the following letter from Miss A. B. 

 Edwards, with permission to publish, of wliich I gladly 

 avail myself.] 



YOV have Lately replied in the pages of Knowledge 

 to certain correspondents who have reminded you 

 that Egyptologists in general regard the Great Pyramid as 

 a tomb only. I am, as you know, one of that number ; 

 but I, nevertheless, gladly take this opportunity of repeat- 

 ing that some of your views as to the astronomical purposes 

 fulfilled at certain stages of its erection, seem to me not 

 only sound and probable, liut proved beyond further ques- 

 tion. Your theory of the u>e and purpose of the water- 



