404' "Jlfr. Stephens on the [Dec! 



hitherto intricate subject has been greatly elucidated by the 

 original researches of Dr. Bostock, who, in the year 1809, was 

 engaged in a series of experiments (the converse of Sir H. Davy's) 

 in search of a vegetable astringent which might serve as a 

 certain test to determine the quantity of gelatine in animal 

 fluids ; during which examination he found so many new sources 

 of error y both in practice and principle, from the use of tan as 

 a test for the quantity of gelatine, that he was compelled to 

 abandon it. As 1 conceive that these objections equally apply 

 to the use of gelatine as a test for the quantity of tan, 1 will 

 here enumerate them, and thus tring into one view the mass of 

 evidence which compels us, however unwillingly, to forego the 

 mode of examination proposed by Sir H. Davy. 



Dr. Bostock discovered that isinglass and glue (in the state 

 in which we generally obtain them) both contain impurities : 

 in isinglass, the insoluble matter sometimes amounts to -.^Vth of 

 the whole ; a circumstance rendering it necessary to separate 

 this pure portion by solution, and resolidify it by evaporation. 

 The glue is a still more uncertain article from the quantity of 

 water it contains, (some pieces dried at 150" Fahr. for 24 hours, 

 indicating so much as lOi per cent.) as well as from the coagu- 

 lated albumen, and muriate of soda which exist in it. Again, 

 Jsinglassand glue differ remarkably in \he:\v powers of concretion: 

 a solution of the former containhig -^'3-th of solid matter would 

 be when cold perfectly co^zcre^e ; whilst a solution of the latter 

 containing an equal weight would (though strongly adhesive) 

 remain c^miQjiuid when cold. 



In his endeavours to procure pure tan. Dr. Bostock found that 

 the extract of rhatany contained it in a state more free from 

 impurities than any vegetable astringent we are acquainted with ; 

 and, therefore, with an infusion of this substance, and the puri- 

 fied isinglass formerly mentioned, he pursued his experiments. 



In addition to the difficulties previously detailed, he found that 

 all the precipitates of tanno-gelatine caught, as directed, on a 

 filter, adhered so strongly to the paper that they could not after- 

 wards be completely separated. Weighing the paper also (before 

 and after) does not remedy this inconvenience, for the strong 

 solutions so thoroughly pervade it, as to defeat all attempts at 

 accuracy. 



But the most striking result obtained by Dr. Bostock is, 

 that the precipitates formed by the gradual mixture of solutions 

 of tan and gelatine, differ in their composition at almost every 

 drop. The first portion of gelatine throws down a solid curd 

 containing 50 per cent, of tan : the ensuing additions form 

 opaque compounds containing less and less of tan, till, at last, 

 the gelatine has so little left to unite with, that it is unable to 

 become a real solid, and thus the imperfect curd last formed 



