84 



♦ KNOWLEDGE • 



[Jan. 19, 18t?. 



constantly found that the richness or quality of a soup 

 depenilod more upon the proper choice of the ingredients, 

 and a proper management of the fire in the combination 

 of these ingredients, than upon the quantity of solid 

 nutritious matter employed ; much more upon the art and 

 skill of the cook than upon the sums laid out in the 

 market." 



A great many fallacies are continually perpetrated, not 

 only by ignorant people, but even by eminent chemists and 

 physiologists, by inattention to what is indicated in this 

 passage. In many chemical and physiological works may 

 be found elaborately minute tables of the chemical composi- 

 tion of certain articles of food, and with these the assump- 

 tion (either directly stated, or implied, as a matter of 

 course) that such tables represent the practical nutritive 

 value of the food. The illusory character of such assump- 

 tion is easily understood. In the first place the analysis is 

 usually that of the article of food in its raw state, and thus 

 all the chemical changes involved in the process of cookery 

 are ignored. 



Secondly, the difficulty or facility of assimilation is too 

 often unheeded. This depends both upon the original con- 

 dition of the food and the changes which the cookery has 

 produced — changes which may double its nutritive value 

 without effecting more than a small percentage of altera- 

 tion in its chemical composition, as revealed by laboratory 

 analysis. 



In the recent discussion on whole-meal bread, for 

 example, chemical analyses of the bran, ic, are quoted, and 

 it is commonly assumed that if these can be shown to con- 

 tain more of the theoretical bone-making or brain-making 

 elements, that they are, therefore, in reference to these 

 requirements, more nutritious than the fine fiour. But 

 before we are justified in asserting this, it must be made 

 clear that these ordinarily rejected portions of the grain 

 are as easily digested and assimilated as the finer inner 

 fiour. 



I think I shall be able to show that the practical failure 

 of this whole-meal bread movement (which is not a novelty, 

 but only a rexdval) is mainly due to the disregard of the 

 cookery question ; that whole-meal prepared as bread by 

 simple baking is less nutritious than tine flour similarly 

 prepared ; but that whole-meal otherwise prepared may be, 

 and has been, made more nutritious than fine white bread. 



Count Rumford supplies us with important data towards 

 the solution of this difficulty. 



Another preliminary example. A pound of bread or 

 biscuit contains more solid nutritive matter than a pound 

 of beefsteak, but does not, when eaten by ordinary mortals, 

 do so much nutritive work. ^^Tiy is this ? 



It is a matter of preparation — not exactly what is called 

 cooking, but equivalent to what cooking should be. It is 

 the preparation which has converted the grass food of the 

 ox into another kind of food which we can assimilate very 

 easily. 



The fact that we use the digestive and nutrient apparatus 

 of sheep, oxen, ic, for the preparation of our food, is 

 merely a transitory barbarism, to be ultimately superseded 

 when my present subject is sufficiently understood and 

 applied to enable us to prepare the constituents of the 

 vegetable kingdom in such a manner that they shall be as 

 easily assimilated as the prepared grass which we call beef 

 and mutton, and which we now use only on account of our 

 ignorance of " The Chemistry of Cooking." 



An international Exhibition of industrial and principally 

 electrical objects is being organised by the Bohemian Ar- 

 chitekten und Ingenieur-Verein, at Prague, to open on 

 March 17. 



PLEASANT HOURS WITH 

 MICROSCOPE. 



THE 



By He.vry J. Slack, F.G.S., F.R.1SLS. 



EVERY possessor of a microscope should exercise its 

 powers upon common things. Many such will exhibit 

 unexpected beauty, and throw light upon very interesting 

 questions. A potato, for example, may be recommended 

 as an excellent subject of investigation ; but as polarized 

 light will be required for some of the obser\-ations, it may 

 be well to say a little about it, lea^-ing it, however, as a 

 subject for some other contributor at a convenient time. 

 " In a beam of ordinary light," says Professor Tyndall, 

 " the vibrations occur in all directions round the line of 

 propagation." Some crj-stalline substances can efTect an 

 important change. Tourmaline, for example, cut in a par- 

 ticular way, will " quench all vibrations save those parallel 

 to its own optical axis. Hence the beam emergent from 

 the plate of tourmaline has all its vibrations reduced 

 to a single plane. In this condition it is a beam 

 of i>lana polarized light." Iceland spar, in the shape 

 of the prisms supplied Vjy opticians, has a like effect, 

 and when light has been polarised by one of them, it will 

 not pass in all directions through a similar prism. When 

 used with the microscope, one prism is placed under the 

 stage to receive light from the mirror, and another just 

 over the objective, or over the eye-piece. If the two 

 prisms cross each other in a certain direction, no polarized 

 light passes ; but if what is called an analysing substance 

 is placed between them, it restores to the light its power 

 of passing through the second prism, and frequently, in so 

 doing, the light waves interfere with each other, and give 

 rise to coloured effects. Polarized light is acted upon by 

 many substances, whose molecules are so arranged that 

 they can vibrate more easily in some directions than 

 others ; and the potato contains one of these in the form 

 of starch. 



First, with a sharp penknife make a very thin slice of 

 the potato, about half-an-inch square, and \-iew it with a 

 half-inch objective and the polarizing apparatus. When 

 the two prisms let the light pass freely, the potato will 

 appear to be composed of small cells of pretty uniform 

 aspect. Turn one of the prisms so as to produce a dark 

 field. If the potato is fiowery, a vast number of little cells 

 will then be brilliantly lit up, while their neighbours are 

 dark. The light ones will be marked by a black cross, and 

 some may show prismatic colours. These cells are starch 

 grains, and constitute the chief value and give the pleasant 

 flavour of a mealy tuber. They are of all sizes, from about 

 a two-hundredth-of-an-inch long, down to tiny ones scarcely 

 visible with the power mentioned. Each starch grain is a 

 little delicate bag, longer than wide, and approximately 

 egg-shaped. The cross is like what might be made by 

 drawing two dark lines at right angles to each other on an 

 egg, beginning at the top of the thickest end. The peculiar 

 action of starch upon the polarized light affords a very 

 ready means of recognition, but it is not the chemical sub- 

 stance, starch, which produces the eflect, but the thin skin 

 or vesicle, and if that is ruptured or destroyed, the cross 

 appears. Another mode of recognising starch is to bring 

 it in contact with a weak solution of iodine, which turns its 

 purplish tending to lilack. 



After trying the potato, other common substances in 

 family use should be examined. Brown it Poison's Corn 

 Flour produces a beautiful eflect with the polarizing prisms. 

 It is nearly all starch — ostensibly of maize. The vesicles 

 are much smaller than the largest of the potato, but give 

 brilliant images. Tous-hs-mois, obtained from a South 



