140 



CHEMISTRY. 



tfrj fa boiled for a sufficient time in water, an additional 

 portion of the same substance* is separated from it. 

 ' So" 9 * *lbumen collects on the surface in the form of 

 scum, accompanied with gome melted fat. The water, 

 when sufficiently concentrated by evaporation, assumes 

 the form of jelly. When evaporated to dryness and 

 treated with alcohol, the gelatine and phosphoric salts 

 :i, while the extractive matter of Thouvenel is dis- 

 solved, and may be obtained by evaporating to dryness. 



The muscle, thus treated with water, is left in the 

 state of grey fibres, insoluble in water, and becoming 

 brittle when dry. This substance possesses all the pro- 

 perties of fibrin. 



From these facts, ascertained by Thouvenel and Four- 

 ITOJ, it appears that the muscles arc composed chiefly 

 ef fibrin, to which they owe their fibrous structuic and 

 their form, and that they contain also, 



1. Albumen. 4. Phosphate of soda. 



2. Gelatine. ~>. Phosphate of ammonia. 



3. Extractive. 6. Phosph. of lime and carb. of do. 



For the discovery of the last ingredients we are indebted 

 to Mr Hatchett, who found that 500 parts of beef mus- 

 cle left, after combustion, a residuum of ti5.fi parts, con- 

 ;iitir.g chiefly of these salts. 



The muscles of different animals differ exceedingly 

 from each other in their appearance and properties, at 

 least as articles of food ; but we know little of their 

 chemical differences. The observations of Thouvenel 

 alone were directed to that object, and they are imper- 

 fect. The flesh of the ox contains, according to him, 

 the greatest quantity of insoluble matter, and leaves the 

 greatest residuum when dried; the flesh of the calf is 

 more aqueous and mucous : the land and water turtle 

 yields more matter to water than the muscle of the ox ; 

 but Thouvenel ascribes the difference to foreign bodies, 

 as ligaments, &c. mixed with the muscle of the turtle : 

 snails yield to water a quantity of matter intermediate 

 between that given by beef and veal ; with them the 

 muscles otjrogt, crayji.ilt, and vipers, agree nearly in 

 this respect ; but the muscles of fresh water fish, not- 

 withstanding their softness, yield a considerably smaller 

 proportion. 



SECT. IV. Of the Skin. 



>kia. The skin is that strong thick covering which enve- 



lopes the whole external surface of animals. It is com- 

 posed chiefly of two parts : a thin white elastic layer on 

 the outside, which is called epidermis or cuticle ; and a 

 much thicker layer, composed of a great many fibres 

 closely interwoven, and disposed in different directions ; 

 thi is called the cutis, or true skin. The epidermis is 

 that part of the skin which is raised in blisters. 



I-.pidermi. 1. The epidermis is easily separated from the cutis by 

 maceration in hot water. It possesses a very great de- 

 gree of elasticity. 



It is totally insoluble in water and in alcohol. Pure 

 fixed alkalies dissolve it completely, as does lime like- 

 wise, though slowly. Sulphuric and muriatic acids do 

 not dissolve, at least they leave no sensible action on it 

 for a considerable tune ; but nitric acid soon deprives it 

 of i;s elasticity, and causes it to fall to pieces. 



If the cuticle be tinged with nitric acid, the applica- 

 tion of ammonia to it is well known to give it instan- 

 taneously a deep orange colour. Now, at Hatchett has 

 shown that this change is also produced upon coagula- 

 ted albumen in the same circumstances, and as the epi- 



tlon of 



dermis rrsi-mbles that substance in all the properties <".. 

 above detailed, it can scarcely be doubted that it is any 1-^imina. 

 thing else than a peculiar modification of coajrulatcd al- 

 bumeo. 



'_'. The cutis is a thick dense membrane, composed of 

 fibres interwoven like the texture of a hat. When it is 

 macerated for sorm- hours in water, and agitation and 

 pressure are employed to accelerate the effect, the blood, 

 and all the extraneous matter with which it was loaded, 

 are separated from it, but its texture remains unaltered. 

 On evaporating the water employed, a small quantity of 

 gelatine may be obtained. No subsequent maceratio'n in 

 cold water has any farther effect. When distilled, it 

 yields the same products as fibrin. The concentrated al- 

 kalies dissolve it, converting it into oil and ammonia. 

 Weak acids soften it, render it transparent, and at last 

 dissolve it. Nitric acid converts it into oxalic acid and 

 fat, while, at the same time, azotic gas and prussic acid 

 are emitted. When heated it contracts, and then swells, 

 exhales a fetid odour, and leaves a dense charcoal, diffi- 

 cult to incinerate. By spontaneous decomposition in 

 water or moiat earth, it is converted into a fatty matter 

 and into ammonia, which compose a kind of soap. When 

 allowed to remain long in water, it softens and putrefies, 

 being converted into a kind of jelly. When long boiled 

 in water it becomes gelatinous, and dissolves completely, 

 constituting a viscid liquor, which, by proper evapora- 

 tion, is converted into glue. Hence the cutis of ani- 

 mals is commonly employed in the manufacture of 

 glue. 



From these facts, the cutis appears to be a peculiar 

 modification of gelatine, enabled to resist the action of 

 water, partly by the compactness of its texture, and 

 partly by the viscidity of the gelatine of which it is 

 formed ; for those skins which dissolve most readily in 

 boiling water afford the worst glue. Mr Hatchett has 

 observed that the viscidity of the gelatine obtained from 

 skins is nearly inversely as their flexibility, the supplest 

 hides always yielding the weakest glue ; but this glue is 

 very soon obtained from them by hot water. The skin 

 of the eel is very flexible, and affords very readily a great 

 proportion of gelatine. The skin of the shark also 

 readily yields abundance of gelatine ; and the same re- 

 mark applies to the skins of the hare, rabbit, calf, and 

 ox ; the difficulty of obtaining the glue and its goodness 

 always increasing with the toughness of the hide. The 

 hide of the rhinoceros, which is exceedingly strong and 

 tough, far surpasses the rest in the difficulty of solution, 

 and in the goodness of its glue. When skins are boiled, 

 they gradually swell and assume the appearance of horn : 

 then they dissolve slowly. 



3. As to the rele mucosum, or the mucous substance, Rete mu- 

 situated between the cutis vera and epidermis, its com- cosum 

 position cannot be determined with precision, because 

 its quantity is too small to admit of examination. It is 

 known that the black colour of negroes depends upon a 

 black pigment, situated in this substance. Oxymuria- 

 tic acid deprives it of its black colour, and renders it 

 yellow. A negro, by keeping his feet for some time in 

 water impregnated with that acid, deprived it of its co- 

 lour, and rendered it nearly white ; but in a few days 

 the black colour returned again with its former inten- 

 sity. This experiment was first made by Dr Beddoes 

 on the fingers of a negro. 



SECT. V. Of Membranes, Tendons, Ligaments, and 

 Glands. 



These substances have not hitherto been subjected to 



