204 



THE POPULAE EDUCATOE. 



LESSONS IN CHEMISTRY. XXIX. 



THE COMPONENTS OF THE ANIMAL FKAME. 



Bones, which, compose the rigid skeleton about which the animal 

 Organism distributes its muscles, nerves, etc., are formed of a 

 dense cellular tissue of gelatine, made stiff by inorganic salts, 

 chiefly phosphates of lime and magnesia and calcic carbonate. 

 This admits of ready proof. Suspend a cleanly-scraped bone in 

 water which has been acidulated with one-sixth of its bulk of 

 hydrochloric acid. Effervescence commences over the whole 

 surface, owing to the escape of the carbonic acid gas. In the 

 course of three or four days, all the mineral salts will have been 

 dissolved out. After several washings in pure water, the bone 

 is found to be perfectly flexible, though it has retained its shape ; 

 if it be dried it will become of a transparent horny appearance, 

 and when boiled for a few hours it will gradually dissolve. The 

 solution on cooling will gelatinise. The following table will 

 show that bones and teeth have a kindred composition : 



Horn. Next in solidity to bones and teeth ranks that horny 

 snbstance which appears in the shells and horns of animals, in 

 aails, wool, hair, etc. It does not dissolve, as the gelatine will, 

 from bones when boiled under pressure, but is softened by heat. 

 It seems to take an intermediate place between gelatinous and 

 albuminoid substances. Alkaline solutions have the power of 

 dissolving it ; when caustic potash is used, ammonia is given 

 off. An idea of the composition of some of these bodies will be 

 given by this table : 



The softer parts of the animal structure comprise various 

 substances, the accurate analysis of which is so difficult that 

 no satisfactory formulte have been devised to express their 

 composition; yet their main properties aro well understood. 

 They are classed under two heads : 



The Albuminoid group, which comprises albumen, fibrine, 

 caseine, and legumine ; and 



The Gelatigenous group, which includes gelatine, chondrine, 

 and osseine. 



Albumen is found in almost a pure state in white of egg, and 

 in the serous fluid of the blood. If white of egg be mixed with 

 water and filtered, the filtrate neutralised exactly with acetic 

 acid, then largely diluted, a copious flocculent precipitate falls, 

 which may be collected on a filter: this is albumen. It is 

 insoluble in water, but readily soluble if the water be slightly 

 alkaline. The most characteristic property of albumen is the 

 fact that it solidifies and becomes insoluble when heated to and 

 above 60 Cent. The solution of albumen in alkaline water 

 does not undergo coagulation when heated ; a small portion of 

 sulphur, however, is separated. 



Alcohol precipitates albumen from its solution; ether does 

 not. The solutions of most of the metallic salts precipitate 

 albumen ; hence in cases of poisoning from such salts, the 

 antidote is the white of eggs. Tannic acid, or infusion of galls, 

 also gives a copious precipitate. In the human body albumen 

 forms 7 per cent, of the blood ; it is found in all serous exuda- 

 tions, but not in any of the fluids with which the -body parts, 

 except in the case of disease. 



Fibrine, like albumen, occurs in a liquid and in a solid state; 

 in the former condition it constitutes 25 per cent, of the blood, 

 and in the latter it forms the largest part of the muscular tissue. 



It may be procured by whipping, with a bunch of twigs, freshly, 

 drawn blood in the act of coagulating, which it always does 

 when removed from the influence of life ; it attaches itself to 

 the twigs in white, opaque, elastic filaments, which under the 

 microscope appear to be composed of small globules arranged 

 in strings. It occurs in muscle in bundles of fibres, hence its 

 name. 



In a solution of very dilute caustic alkali, fibrine is soluble. 



Caseine constitutes nearly 3 per cent, of milk, and forms the 

 bulk of all cheese. Like albumen, it is insoluble save in alkaline 

 solutions. In milk it is in its soluble state, owing to the 

 presence of a little free alkali. If this be neutralised by a little 

 sulphuric acid, the milk " curdles," the coagulum is collected, 

 re-dissolved by the aid of a little sodium carbonate, and allowed to> 

 stand until the fats and oils rise, when the liquid containing the 

 caseine is drawn off by a siphon ; to this acid is again added, and 

 the caseine again precipitated ; this process is repeated several 

 times, and finally it is treated with ether to remove all traces 

 of fat. 



It appears as a white curdy substance. When dissolved in a 

 weak alkaline solution, as it is in milk, caseine does not coagu- 

 late upon boiling ; by this means it may be distinguished from 

 albumen ; but when its solution is hot, it absorbs oxygen from 

 the air, and becomes covered with a skin. The most remarkable 

 action caseine undergoes is produced by the secretion from the 

 mucous membrane of the stomach. This is taken advantage of 

 to make cheese. The inner membrane of the fourth stomach 

 of a calf is salted and dried ; when the milk is slightly warmed 

 and brought in contact with this, rennet, the caseine coagulates, 

 and carries down with it all the fatty compounds in the milk, 

 leaving a clear liquid, whey. This curd, submitted to strong 

 pressure, becomes cheese. 



Legumine is the form in which azotised matter appears in the 

 vegetable kingdom. It is so closely similar to caseine that many 

 eminent chemists pronounce them to be the same body. It 

 appears in the seeds of leguminous plants. To prepare it, digest 

 the pulp of crushed peas in warm water for two or three hours. 

 The undissolved portion is separated by straining, the liquid 

 is allowed to deposit its starch, then, on the addition of a littlo 

 acetic acid, the legumine falls as a flocculent precipitate. It 

 behaves like albumin, and even undergoes coagulation by rennet, 

 which fact enables the Chinese to make a kind of cheese from 

 dried peas, which contain a fourth of their weight of legumine. 



The relative composition of these albuminoid substances will 

 be indicated in the following table : 



ALBUMINOID GROUP. 



Proteine. When a solution of any albuminoid substance is 

 neutralised a flocculent precipitate falls, which proves to be the 

 same for each of the four members of the group. At first this was 

 considered to be the base of the group, the members being ita 

 compounds with sulphur and phosphorus. Hence it was called 

 proteinc, from irpunos, first the foundation of the group. But 

 this idea has been abandoned, and proteine is considered to be 

 a product of the action of the alkali on an albuminoid sub- 

 stance. 



The Gelatigenous group. These substances contain less 

 carbon and sulphur, but more nitrogen than the preceding group, 

 and have no representative in the vegetable kingdom. 



Gelatine is the main component of the walls of the cells which, 

 filled with albuminoid matter, build up the material of the animal 

 frame ; it is, as we have noticed, the organic constituent of 

 bone. 



Gelatine is found in almost a pure state as isinglass, which is 

 prepared from the floating bladder of the sturgeon. Glue and 

 size are impure varieties. It is soluble in hot water ; corrosive 

 sublimate, platinic chloride, and infusion of galls precipitate it. 



