GLUE 683 



exposed a serious deterioration ; the damp condensed upon its surface occasioning a 

 general mouldiness. A thunderstorm sometimes destroys tho coagulating power in 

 the whole laminae at once ; or causes the glue to turn on the nets, in the language 

 of the manufacturer. A wind too dry or too hot may cause it to dry so quickly, as to 

 prevent it from contracting to its proper size without numerous cracks and fissures. 

 In this predicament, the closing of all the flaps of the windows is the only means of 

 abating the mischief. On these accounts it is of importance to select the most tem- 

 perate season of the year, such as spring and autumn, for the glue-manufacture. 



After the glue is dried upon the nets it may still preserve too much flexibility, or 

 softness at least, to be saleable ; in which case it must be dried in a stove by arti- 

 ficial heat. This aid is peculiarly requisite in a humid climate, like that of Great 

 Britain. 



When sufficiently dry it next receives a gloss, by being dipped, cake by cake, in hot 

 water, and then rubbed with a brush, also moistened in hot water ; after which the 

 glue is arranged upon a hurdle, and transferred to the stove-room, if the weather be 

 not sufficiently hot. One day of proper drought will make it ready for being packed 

 up in casks. 



The pale-coloured, hard, and solid article, possessing a brilliant fracture, which is 

 made from the parings of ox-hides by the first process, is the best and most cohesive, 

 and is most suitable for joiners, cabinet-makers, painters, &c. But many workmen 

 are influenced by such ignorant prejudices, that they still prefer a dark-coloured 

 article, with somewhat of a fetid odour, indicative of its impurity and bad preparation, 

 the result of bad materials and too long exposure to the boiling heat. 



There is a good deal of glue made in France from bones freed from the phosphate 

 of lime by muriatic acid. This is a poqr article, possessing little cohesive force. It 

 dissolves almost entirely in cold water, which is the best criterion of its imperfection. 

 Glue should merely soften in cold water, and the more considerably it swells, the 

 better, generally speaking, it is. 



Some manufacturers prefer a brass to a copper pan for boiling glue, and insist much 

 on skimming it as it boils ; but the apparatus represented renders skimming of little 

 consequence. For use, glue should be broken into small pieces, put along with some 

 water in a vessel, allowed to soak for some hours, and subjected to the heat of a 

 boiling-water bath, but not boiled itself. The surrounding hot water keeps it long in 

 a fit state for joiners, cabinet-makers, &c. 



Water containing only one-hundredth part of good glue, forms a tremulous solid. 

 When the solution, however, is heated and cooled several times, it loses the property 

 of gelatinising, even though it be enclosed in a vessel hermetically sealed. Isinglass 

 or fish-glue undergoes the same change. Common glue is not soluble in alcohol, but 

 is precipitated in a white, coherent, elastic mass, when its watery solution is treated 

 with that fluid. By transmitting chlorine gas through a warm solution of glue, a 

 combination is very readily effected, and a viscid mass is obtained like that thrown 

 down by alcohol. A little chlorine suffices to precipitate the whole of the glue. 

 Concentrated sulphuric acid makes glue undergo remarkable changes ; during which 

 are produced sugar of gelatine, leucine, an animal matter, &c. Nitric acid, with the 

 aid of heat, converts glue into malic acid, oxalic acid, a fat analogous to suet, and into 

 tannin ; so that, in this way, one piece of skin may be made to tan another. When 

 the mixture of glue and nitric acid is much evaporated, a detonation at last takes place. 

 Strong acetic acid renders glue first soft and transparent, and then dissolves it. Though 

 the solution does not gelatinise, it preserves the property of glueing surfaces together 

 when it dries. Liquid glue dissolves a considerable quantity of lime, and also of the 

 phosphate of lime recently precipitated. Accordingly glue is sometimes contaminated 

 with that salt. Tannin, both natural and artificial, combines with glue; and with such 

 effect, that one part of glue dissolved in 5,000 parts of water affords a sensible preci- 

 pitate with the infusion of nutgalls. Tannin unites with glue in several proportions, 

 which are to each other as the numbers 1, 1, and 2 : one compound consists of 100 

 glue and^SQ tannin; another of 100 glue and 60 tannin ; and a third of 100 glue and 

 120 tannin. These two substances cannot be afterwards separated from each other by 

 any known chemical process. 



Glue maybe freed from the foreign animal matters generally present in it, by soft- 

 ening it in cold water, washing it with the same several times till it no longer gives 

 out any colour, then bruising it with the hand, and suspending it in a linen bag beneath 

 the surface of a large quantity of water at 60 F. In this case, the water loaded with 

 the soluble impurities of the glue gradually sinks to the bottom of the vessel, while 

 the pure glue remains in the bag surrounded with water. If this softened glue be 

 heated to 92 without adding water, it will liquefy ; and if we heat it to 122, and 

 filter it, some albuminous and other impurities will remain on the filter, while a 

 colourless solution of glue will pass through. ... 



