LESSONS IN CHEMISTRY. 



241 



LESSONS IN CHEMISTRY. XXX. 



MILK-THE EXCRETIONS FOOD-VEOET \ 

 COMPONENTS. 



Milk. Boeing that thia fluid is the only food of the young 

 of tho mammalia for many months, it may be regarded an the 

 most important as well aa the moat peculiar of the secretion*. 



. 'iily Hocrotcd by the fomalo, who for thia purpose in en- 



with iiiivminillnry glands. It consiata of caaeine, which, 



aa wo have noon, in analogous to muaole ; of fatty principle* ; a 



ir sugar, and sundry salts. Its whiteness is an optical 

 ,i, -lii-ion ; under the microscope it is a clear liquid, in which 

 small globuloa of transparent fatty matter, enclosed in a slight 

 l>illirlo, float. If allowed to stand, these rise to the surface, 

 forming cream. When this cream ia churned, the pellicle of the 

 fat-globule is mechanically broken the fat agglomerate*, 

 forming butter. The butter is washed with cold water, in order 



> >ve as much as possible the casoine, which, owing to its 

 complicated composition, readily decomposes, and thua makes 

 tho butter rancid. Salt is also added to retard the decompo- 

 sition of the fat. Tin- casoine, which is in solution in the butter- 

 milk, has been described. Albumen is also found in colostrum, 

 that is. milk which in secreted 

 soon after tho birth of tho young. 

 The milk of the cow contains 

 so much albumen that it 

 coagulates when it is boiled. 

 The sugar in the milk has tho 

 peculiar quality of not under- 

 going alcoholic fermentation, 

 and the consequent liberation 

 of carbonic acid gas. Such a 

 product would be dangerous to 

 the tender stomach and intes- 

 tines of tho young animal. 

 The Arabs prepare a fermented 

 liquor from mare's milk, but in 

 this casea part of the milk-sugar 

 is converted into grape-sugar 

 by the caseine, and becomes 

 the source of the fermentation. 

 When in a warm atmosphere 

 the sugar is liable to undergo 

 a change, and the product is 

 lactic acid ; and seeing that 

 caseine is insoluble in the 

 presence of an acid, it is pre- 

 cipitated as curds ; thus tho milk is said " to turn." The com- 

 position of cow's milk will be seen from the following analysis : 



Water 873-00 



Butter 30-00 



Caseine 48*20 



Milk-sugar 43 '90 



Calcium phosphate . . . 2 '31 



Magnesia 0'42 



Iron 0-07 



Potassium chloride . . . 1*44 



Sodium 0'24 



Soda with caserne 0*42 



STARCH : Fig. 55, Starch of Wheat ; Pig. 56, Starch of Arrowroot 

 Fig. 57, Starch of Bice. 



1000-00 



Urine, tho chief of the fluid excrements, is separated by the 

 kidneys from arterial blood. Its contents, which ore subject 

 to much variation, may be thus given : 



Water 933'00 



. 30-10 

 17-14 



Urea .... 

 Extractive matter. 

 Uric acid . . 

 Sodium Sulphates ) 

 Potassium Sulphates j 

 Sodium Phosphate 

 Ammonium . 

 Calcium and Magnesium 

 Sodium Chloride . 

 Sal-ammoniac 

 Silica .... 

 Mucus . . . 



1-00 

 6-87 



2-94 

 1-65 

 1-00 

 4-45 

 1-50 

 0-03 

 032 



1000-00 

 This excretion is tho channel which carries off the wasto mate- 



94 N.E. 



rial from the wear and tear of the body. It b an amber, 

 coloured liquid, poMuring an aromatic odour when warm, 

 except in di*ei*e ; and during the proaes* of diyettton U b Mid. 

 Urea MM ., principal outlet for the nitrogen of 



the system. A human adult excrete* about an ounce and a 

 quarter daily. It may be separated from nrine by adding an 

 equal bulk of nitric acid, and treating the adeoUr eryvtal* of 

 nitrate of urea thus obtained with baric carbonate. The carbonic 

 acid cornea off, and the urea cryiitallUe* from the barium nitrate. 

 Since ita formula may be written thai, NH 4 O,CK, 

 sometimes called Oyanate of Ammonia. This a*lt can be mad* 

 readily, and when a volution of it U evaporated at a jenUs 

 heat, it undergoe* a metamorphonU, and become* area 

 without any change in it* element*, which *eem to have grouped 

 themselves anew. It will be noticed that an addition of two 

 molecule* of water will produce ammonium carbonate, and 

 thia explain* the can*e of the fact that no urea i* found hi 

 decomposing urine, but a quantity of ammonium carbonate. 



Uric or Lithic Acid (H ? C 5 H t N 4 O,). Human mine doe* not 

 contain much of thia acid, but it i* present in considerable 

 quantity in the semi-liquid excrement of bird* ; and the vast 

 beds of guano chiefly consist of ammonium nrate. When urine 



i* anrcharged with this acid, 

 it deposit* itwslf in hard 

 crystallised grain*, forming red 

 gravel. If thia deposition go 

 on in the kidney or bladder, 

 calculi are the result. In gouty 

 subject* it i* liable to ac- 

 cumulate, joined wLh soda, 

 about tho joints of the finger* 

 and toes, forming what are 

 improperly called chalk lionet. 

 It ia found in combination 

 with the alkalies, and theae 

 urates form the deposit* which 

 urine frequently give* on cool- 

 ing. There are many other 

 compound* which are found in 

 minute quantities in urine, but 

 which, although of great che- 

 mical use, are not of general 

 interest. 



TUB NUTRITION OP ANIMALS 

 AND PLANTS. 



In order to supply the body 

 with the various compound* of 



which we have found it is composed, we take food that ia, 

 vegetable and animal substances which contain theee requisite 

 bodies. We need say nothing of animal food, for the consti- 

 tution of all flesh is analogous ; but the existence of the com- 

 pounds which build up the animal frame in the vegetable king- 

 dom is not so evident. Nutritious substances may be divided 

 into two great classes those which contain nitrogen, or atotited 

 bodies; and those which are wanting in nitrogen, or noft-asohwd 

 bodies. The former supply the flesh with building material, for 

 muscle, etc., contains nitrogen ; and the hitter seem chiefly 

 to devote themselves to the maintenance of animal heat in the 

 system. 



Corn. Taking corn a* the staple vegetable food of man, we 

 find ita constituent* are : 1, atarch ; 2, gluten, the albumen of 

 the vegetable kingdom ; 3, dextrine ; 4, oily matter ; 5, saline 

 matter ; and 6, a skeleton of ligneous tisane, which nndergoM 

 no change in passing through the system. The proportion of 

 these ingredients varies in different climate*. The following* 

 table gives some idea of this : 



