CHEMISTRY. 



[DIGESTION AHD HUTRITIOH. 



condiment*. None of the*e ectiou* need any further 



\Vo must, howerer, guard against one error, not 

 eldom met with. In Cohering that *olid food only 

 atkfie* the hunger, while liquid* only quench the thirst, 

 we forget that there U only one alimentary principle 

 namely, water that affect* that condition of the blood 

 which bring* on thirst But water it contained in all 

 aliments in *uch abundance, that, on an average, it con- 

 stitute* more than half of their weight All beverages, 

 on the other hand, contain also some other alimentary 

 principle* than water; for oven in what we are accus- 

 tomed to call pure water, there are always some of the 

 compound* of chlorine and salt* ; while in milk, we have 

 all ela**e* of simple alimentary substances together, it 

 being composed of water, compounds of chlorine, salts, 

 cawine, fat, sugar, <tc. 



The condiment* are also, in the restricted sense in 

 which we shall employ the word, principally composed of 

 alimentary principle* ; only a few of them contain sub- 

 stance* which, although exciting the gustatory nerves, 

 and stimulating the digestive organs to a greater activity, 

 may not also be considered a* restoring some of the losses 

 of the body. 



J 39. As the alimentary principles are not found in 

 nature in their simplest form, so also none of them is 

 sufficient of itself to nourish the body. Not even a 

 variety of these substances, if belonging only to one of 

 the three group* before-mentioned, is able, if exclusively 

 eaten, to maintain life : we may go further, and add 

 that life cannot be sustained by two of these sections, if 

 the third be lacking. 



Not sugar, salts, nor albumen, if taken without any 

 *ub*tances of the two other groups, are able to destroy 

 the consequences which the change of matter effects, 

 when the process of excretion goes on, without the 

 accompanying one of nutrition. 



Without phosphate of lime the bones cannot be formed, 

 whatever quantity of albumen and fat we may consume ; 

 no muscular tissue could grow without albumen, hov. 

 we might overload the stomach with sugar and salts ; and, 

 finally, without fat, no brain could be formed. But the 

 bones, the brain, and the muscles are the most essential 

 organs of the human body. 



No element can be transformed into another : this is 

 the whole solution of the secret. Phosphorus does not 

 turn into oxygen, nor oxygen into carbon, nor carbon 

 into nitrogen, nor nitrogen into sulphur : no power short 

 of the miraculous is able to effect an exception from 

 this rule. As something cannot be created from nothing, 

 no power of tho body is able to transform iron into hydro- 

 gen, or chlorine into calcium. 



The organic alimentary principles, destitute of nitrogen, 

 cannot be changed into those containing nitrogen, nor 

 the nitrogeiused organic substances into inorganic salts, 

 containing any other elements than nitrogen, carbon, 

 hydrogen, oxygen, sulphur, and phosphorus. 



It might be supposed that albumen could be trans- 

 formed into fat, because albumen, as well as fat, contains 

 carbon, hydrogen, and oxygen ; it could also be suggested 

 that carbonates of alkalies and water, as containing 

 carbon, hydrogen, and oxygen, could form sugar : but 

 experience ha* shown that tho animal body cannot ac- 

 complish tho former transformation in a sufficient quan- 

 tity, nor tho latter at all 



Complete substitutes, therefore, for the nutriments 

 which maintain the life constantly, can only be formed by 

 a combination of all three group* of the alimentary prin- 

 ciple*. 



40. Tut* of ]>'i,jr,til.Hity. The more easily these 

 alimentary principles are dissolved in tho digestive fluids, 

 and changed into the constituent* of tho blood, 

 greater i* their digestibility ; for digestion consists not 

 in the solution of bodies only, but also in their trans- 

 formation into tho essential constituent* of tho blood. 

 Both condition* are of equal importance. 



If two imbalance*, therefore, are dissolved with equal 

 MM, that will be the more digestible which lias th. 

 greater similarity to some constituent of the blood. 



Stearin.- and margarine, for instance, are almost equally 

 soluble in tho digestive juices ; but as margarine is 

 sent in the blood, and stearine is not, it follows that 

 margarine surpasses stearino in digestibility. 



But if tho conformity of two alimentary principle 

 with the constituents of the blood be equal, the m< in- 

 soluble is the more digestible. Soluble album. : 

 tibriiio stand equally near to the blood, both Iwini; 

 tained in it. As the soluble album. r, is more 



readily dissolved in the digestive juices than tibrino, the 

 digestion of the latter is more difficult. 



Hence it results, that tho difficulty with which ali- 

 mentary principles are dissolved, is, in many cases, com- 

 pensated by their conformity with tho ingredients of tho 

 blood. Although gum, for example, is much more easily 

 dissolved than fat, the latter, if not taken in too great a 

 quantity, is digested by a healthy stomach, under certain 

 circumstances, more readily than gum ; for gum U not 

 contained in the blood, while fat belongs to its essential 

 constituents. Gum has first to be transformed into 

 sugar, then into lactic acid, and finally into butyric acid 

 and some other fatty substances ; while fat, on the other 

 hand, as containing oleine and margarine, is represented 

 in the blood itself. In the formation of fat from starch, 

 the latter must first be changed into gum, then into 

 sugar; the sugar into lactic acid, and this again into 

 butyric acid. It hence appears, that of all these sub- 

 stances lactic acid is the most digestible, even if all \\< \-- 

 equally soluble in water ; sugar would be next to it, then 

 gum, and finally starch. Lactic acid and sugar are, be- 

 sides, more soluble than gum, and gum than st 

 The latter is, therefore, for two reasons, the least <( 

 tible of the above-mentioned alimentary principles. It 

 follows, again, from the same law, that a substance which 

 is both more easily dissolved in water than another, and 

 more readily transformed into the constituents of tho 

 blood, greatly surpasses the latter in digestibility. 



Of the aliments, those are the most digestible which 

 contain the greatest number of alimentary prim 

 easily soluble, and easily turned into the constituents of 

 the blood. 



41. Tests of Nutritiveneis. The nutritiveness of any 

 kind of solid or liquid food depends upon three circum- 

 stances : its digestibility, the quantity, and tho propor- 

 tions of the alimentary principles contained in it. 



If an aliment contain many indigestible substances, 

 which are voided again in an undissolved state with tho 

 excrements, it must lose as much of its nutritiveness ; 

 for only that which passes as an essential constituent into 

 the blood, is to be considered as an alimentary principle. 

 The more digestible an aliment is, therefore, other things 

 being equal, the more nourishing it 1 



In testing the value of a nutriment with respect to tho 

 alimentary principle* contained in it, no regard is paid 

 to the amount of water contained. Water is, in gen. 

 so easily to bo procured, that wo have not to take it into 

 consideration, when judging of the value of solid or liquid 

 food, with respect to its nutritious properties. In a dry 

 arid desert, however, water would become tho most im- 

 portant nutriment; and an aliment containing much 

 water would be the most nutritious food. V 

 deficiency in water exists, that, aliment is the most 

 nutritious which contains tho greatest proportion of ali- 

 mentary principles, and conveys, therefore, to tho blood 

 the greatest quantity of its essential constituent*. 



Besides the digestibility and the abundance of solid 

 constituents, the combination of an aliment is also of tho 

 greatest importance. As tho blood contains a gr< 

 proportion of albumen than of salts, and more salts than 

 fat, tho proportion of those alimentary principles will 

 determine whether any particular food be nutritious 

 or otherwise. Just a* alimentary principles of eipial 

 .-.olubility are tho more digestible, tho more completely 

 they correspond with tho several constituents of tho 

 blood ; so an aliment is generally mot.' nutritions, if tho 

 c. 'iid. inatiou of its alimentary principles correspond 

 more exactly with the composition of tho blood. Thus, 

 nutritious aliment is characterised by containing, of 

 uitrogcnised substances, more organic than inorganic, 



