GELATIN. 



405 



it is obtained previous to the action of boiling 

 water, and, moreover, as it does not occur 

 in any of the animal fluids or secretions, it 

 has been regarded by some chemists, and 

 especially by Berzelius, as a product of the 

 action of water and heat, and not as a mere 

 educt. He compares its formation to the 

 conversion of starch into gum and sugar, and 

 remarks that in both cases the change is ac- 

 celerated by the presence of dilute acids. 



Pure gelatin is colourless, transparent, in- 

 odorous, insipid, and neither acid nor alkaline ; 

 heat softens it and exhales a peculiar odour, 

 and it burns with smoke and flame, leaving 

 a bulky coal, difficult of incineration and 

 containing phosphate of lime : it yields much 

 ammonia, and the other ordinary products of 

 analogous animal compounds, when subjected 

 to destructive distillation. 



In cold water dry gelatin swells and be- 

 comes opaque, and when gently heated it 

 dissolves and forms a clear colourless solution, 

 which gelatinises when cold. According to 

 Dr. Bostock, one part of isinglass to 100 

 parts of water yields a perfect jelly, but with 

 180 of water it does not concrete.* Those 

 modifications of gelatin which are the least 

 soluble in hot water yield the strongest jelly. 

 When the same portion of jelly is repeatedly 

 liquefied and cooled, it gradually loses the 

 property of gelatinising, and becomes so far 

 modified as to leave a brownish gummy residue 

 when evaporated, which readily dissolves in 

 cold water. L. Gmelin kept a solution of 

 isinglass in a sealed tube for several weeks 

 at the temperature of 212 : it was thus 

 changed to the consistency of turpentine, was 

 deliquescent, soluble in cold water, and par- 

 tially so in alcohol. 



An aqueous solution of gelatin exposed for 

 some time to the air at the temperature of 

 60 to 70 becomes at first thinner and sour, 

 and afterwards ammoniacal and fetid : the 

 addition of acetic acid prevents the putrefac- 

 tion without impairing the adhesive power of 

 the gelatin. 



Gelatin is insoluble in alcohol and ether, 

 and in the fixed and volatile oils. When a 

 strong aqueous solution of gelatin is dropped 

 into alcohol, it forms a white adhesive and 

 elastic mass, which adheres strongly to the 

 glass, and which, like dry gelatin, softens, but 

 does not dissolve in cold water. 



When chlorine is passed through a warm 

 and somewhat concentrated solution of gelatin, 

 each bubble becomes covered with an elastic 

 film, and deposits, on bursting, a white, tough 

 viscid matter ; the whole of the gelatin is thus 

 precipitated, and free muriatic acid is formed. 

 This chloride of gelatin is insoluble in water 

 and alcohol, and remains acid, and smells 

 of chlorine, even after it has been kneaded 

 in warm water. Dissolved in caustic am- 

 monia in a tube over mercury, it evolves 

 nitrogen and becomes mucilaginous. It is 

 soluble in acetic acid; but the solution, though 

 rendered turbid by dilution, gives no preci- 



* Nicholson's Journal, xi. 244. 



pitate by ferro-cyanuret of potassium, so that 

 the gelatin is not thus converted into albumen, 

 No analogous compound is produced either 

 by iodine or bromine. 



The action of sulphuric acid on gelatin 

 has been studied by Braconnot.* When one 

 part of glue and two of sulphuric acid are 

 mixed, they form in twenty-four hours a clear 

 fluid, which, when diluted with eight parts 

 of water, boiled for eight hours, (the loss by 

 evaporation being replaced by fresh portions 

 of water,) and then neutralised by chalk, 

 filtered, evaporated to the consistency of syrup, 

 and set aside for a month, yields a crystalline 

 crust of a peculiar saccharine substance, which 

 is insoluble in alcohol and ether, unsusceptible 

 of vinous fermentation, and gives ammonia 

 by destructive distillation. It combines and 

 forms a peculiar crystallisable compound with 

 nitric acid, which he calls the nitro-saccharic 

 acid, and which combines with the salifiable 

 bases and forms distinct salts, the properties 

 of which closely resemble the carbazotates. 



Dilute nitric acid dissolves gelatin without 

 the evolution of nitrous gas, and forms a yellow 

 solution, which, by evaporation, (or the ad- 

 dition of an alkali,) becomes darker, and at 

 last evolves nitrous gas, and passes (often with 

 ignition) into a spongy coal.f By concen- 

 trated nitric acid gelatin is converted into 

 malic and oxalic acids, a fatty substance, and 

 artificial tan.]: 



Acetic acid dissolves gelatin and the solution 

 does not gelatinise, but upon drying, the ad- 

 hesive power of the gelatin is unimpaired: 

 the dilute acids do not generally prevent ge- 

 latinisation. 



Neither the dilute caustic alkalis nor am- 

 monia prevent the concretion of a solution of 

 gelatin, but they render it turbid by precipi- 

 tating its phosphate of lime. Gelatin is soluble 

 in strong caustic potash, with the exception 

 of a residue of phosphate of lime. The solu- 

 tion, when neutralised by acetic acid, does 

 not gelatinise, and yields on evaporation a 

 compound of gelatine with acetate of potash, 

 which is soluble in alcohol. Sulphuric acid 

 precipitates sulphate of potash from this acetic 

 solution, in combination with gelatin ; and 

 this compound precipitate, dissolved in water, 

 crystallizes by spontaneous evaporation to the 

 last drop. 



Hydrate of lime does not affect a solution 

 of gelatin, but much lime is dissolved by it : 

 it also takes up a considerable quantity of 

 recently precipitated phosphate of lime. 



Gelatin is not precipitated by solution of 

 alum, but when an alkali is added the alumine 

 falls in combination with gelatin. The alu- 

 minous solution of gelatin is used for sizing 

 paper, and for communicating to woollen 

 cloth a certain degree of impenetrability to 

 water. 



The acetates of lead do not precipitate pure 

 gelatin; by corrosive sublimate its solution is 



* Annales de China, et Phys. xiii. 



t Hatchett, Phil. Trans. 1800, p. 369. 



\ Ibid. 



6 Berzelius. 



