DR DUNCAN on Mudarine. 439 



altered by heat, so that it has become no longer soluble in the 

 menstruum, either by restoring the original temperature, or by in- 

 creasing the quantity of the menstruum. Thus albumen, once 

 coagulated by heat, is rendered permanently insoluble in water. 

 In all other instances, the solubility of organic principles is sup- 

 posed to be increased by increase of temperature. By heating 

 the menstruum, it commonly acts more quickly and more com- 

 pletely, the soluble principles are more speedily extracted from or- 

 ganic compounds, and in larger quantity, and the solution is more 

 liquid and perfect ; while, on the contrary, on cooling, the prin- 

 ciple dissolved separates from a hot saturated solution either by 

 precipitation or crystallization, or by becoming viscid, or forming 

 a jelly, and in all these cases the principle thus separated is re- 

 dissolved by again increasing the temperature, shewing that its 

 nature is not altered, and that they are simple examples of the 

 rule that the solubility of bodies is increased by increase of tem- 

 perature. The relative solubility of animal gelatine and of pec- 

 tic acid, at different temperatures, may be specified with peculiar 

 propriety as forming a striking contrast with the subject of this 

 paper. Gelatine is sparingly soluble in water at the ordinary 

 temperature of the atmosphere, so that cold water is incapable 

 of extracting it from bones, horn, membranes, tendons, or even 

 flesh. By increase of temperature, it becomes rapidly more so- 

 luble, and most of these substances yield it very readily to boil- 

 ing water. Nay, by increasing the temperature of water above 

 the boiling point in PAPIN'S Digestor, it becomes progressively 

 still more soluble ; and, accordingly, this method is employed by 

 D'ARCET to extract gelatine from the hardest bones. On cool- 

 ing, the water is no longer capable of retaining the whole gela- 

 tine dissolved, and the solution, by reduction of temperature, 

 forms a tremulous jelly, more or less solid in proportion to its 

 concentration, which is again readily dissolved into a fluid by the 

 application of heat, properties very nearly the reverse of those 



