PROTEIN. 



167 



of their composition must rentier such meta- 

 morphoses comparatively easy. 



The composition of fibrin is C 400 H 310 

 N 50 O 120 SP, or ten equivalents of protein 

 united to one of sulphur and phosphorus. 

 It also usually contains from 1'3 to 2'3 per 

 cent, of inorganic matter, chiefly phosphate 

 and sulphate of lime, and alkaline salts. 



Albumen. This important compound, so 

 called from its constituting the solid matter of 

 white of egg, exists in two conditions, perfectly 

 distinct in physical properties from each other ; 

 the one soluble and miscihle with water in all 

 proportions, as it is found in the serum and 

 white of egg ; the other solid, and quite inso- 

 luble in water, as in white of egg after boiling. 

 The solid form is also met with, in a some- 

 what modified condition, in the albuminous 

 tissues of the body, as the brain, spinal cord, 

 nerves, &c. The proportion of albumen con- 

 tained in some of the animal products may be 

 seen in the following table. 



100 jmrtif. Albumen. 



Blood of ox 18-6 



hog 18-58 



goat 19-28 



sheep 18-35 



East India isinglass 7'2 to 135 



Egg, white of 15'5 



yolk of 17-47 



Liver of ox, parenchyma of 20'19 



Sweetbread (thymusj of calf 14*0 



Muscle of beef 2'2 



veal 3-2 



pork 2-6 



roedeer 2-3 



pigeon 4'5 



chicken 3'0 



carp 5'2 



trout 4-4 



Brain 7'0 



Optic nerve 22'0 



Albumen, in a state of absolute purity, has 

 been but imperfectly examined. It may be 

 prepared by the following process, recently 

 adopted by Wurtz. A quantity of white of 

 egg is well beaten up with about twice its bulk 

 ot water, and strained through linen to separate 

 the cellular membrane. A solution of subace- 

 tate of lead is cautiously added, which throws 

 down a copious precipitate ; but care must be 

 taken to avoid adding an excess of the preci- 

 pitant, which would partly redissolve it. The 

 precipitate should be well washed, and while 

 suspended in water a stream of carbonic acid 

 passed through it : the liquid soon becomes 

 frothy, owing to the decomposition of the albu- 

 minate of lead and liberation of free albumen, 

 carbonate of lead being precipitated. The solu- 

 tion of albumen, after filtration, generally con- 

 tains a trace of oxide of lead, which may be 

 separated by adding a few drops of solution of 

 sulphuretted hydrogen, and warming the liquid 

 till it just begins to coagulate, when the whole 

 of the sulphuretoflead is entangled in the coa- 

 gulum : the liquid, which after another filtration 

 is clear and transparent, should be cautiously 



evaporated at a temperature not exceeding 

 120, when it leaves a residue of pure albumen. 



Albumen thus prepared is brittle, semitrans- 

 parent, without taste or smell, and almost 

 colourless. When burnt it leaves a very minute 

 quantity of inorganic residue, which seems to 

 be quite free from alkali : this fact is important, 

 as it tends to settle a question which has been 

 long disputed, viz. whether pure albumen is 

 really soluble in water, or whether its solubility 

 is due to the free alkali with which it is usually 

 associated. If dry albumen be digested with 

 water in a moderately warm place it readily 

 dissolves, but a small insoluble residue always 

 remains. According to Wurtz a solution of 

 pure albumen begins to coagulate when heated 

 to about 140; but if it be perfectly dry it 

 may be raised to 280 or 290 without losing 

 its solubility. It appears to have a slightly 

 acid reaction, and if digested at a gentle heat, 

 with a solution of carbonate of soda, it dis- 

 places the carbonic acid and combines with the 

 soda. The albumen contained in white of 

 eggs is composedof C 400 H 31 N 60 O 1 20 SP, 

 or ten equivalents of protein plus one equiva- 

 lent of sulphur and phosphorus ; while that 

 obtained from the serum contains an additional 

 equivalent of sulphur, or C 400 H 310 N so 

 O 120 S 2 P. It is usually associated with from 

 two to five per cent, of inorganic salts. 



The appearances presented by albumen with 

 reagents are in most cases very similar to those 

 of protein, which I have already described, 

 and its solution in hydrochloric acid has the 

 characteristic blue colour. Most of the acids 

 precipitate it from its solution, but this is not 

 the case with tartaric, acetic, and tribasic phos- 

 phoric acids. Hence nitric acid is often used 

 to detect albumen in the secretions. Another 

 delicate test for albumen is ferrocyanide of 

 potassium, which gives a white precipitate even 

 with acid solutions; the ferridcyanide of potas- 

 sium gives a yellowish precipitate. The appli- 

 cation of heat is also a good test for this 

 principle: but as the presence of free alkali 

 tends to prevent its coagulation, it is always ad- 

 visable to add at the same time a drop or two 

 of nitric acid, when, if both cause a precipitate, 

 the presence of albumen may be considered 

 certain: it must be remembered too that the 

 presence of those acids which do not precipi- 

 tate albumen, such as the tribasic phosphoric, 

 tartaric, and acetic, also interferes with its 

 coagulation by heat. Many of the metallic 

 salts, when added to albumen, form insoluble 

 precipitates, which are in most cases compounds 

 of albumen with the acid or the base of the 

 salt. A drop of a solution of bichloride of 

 mercury will thus indicate the presence of 

 albumen, even when diluted with two thousand 

 times its weight of water; and this property of 

 forming an insoluble compound has been taken 

 advantage of in the treatment of cases of poi- 

 soning with the bichloride, when the white of 

 egg has been found of great service ; the white 

 of one egg being sufficient, according to the 

 experiments of Peschier, to neutralize the 

 effects of four grains of the poison. Albumen 

 is precipitated from its solutions by many 



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