1825.] comparative Tantiing Powers of Astringents, 405 



(being nearly all gelatine) remains suspended through th© 

 fluid. 



This one fact is sufficient to invalidate the whole process, and 

 all the calculations founded on it, respecting the quantity of tan- 

 present in any solution ; for they rested entirely on the presump- 

 tion that tan and gelatine always combined in one proportion 

 only ; whereas it appears from Dr. B.'s researches, that they are 

 capable of uniting in several: gelatine combining chemically 

 with an equal weight of tan, if within its reach, and also influenced 

 so strongly by a smaller portion, though the union here may be 

 somewhat mechanical^ as to leave its solution in water to unite 

 with it. (Vide Nicholson's Journal, vol. 24, " On the Union of 

 Tan and Jelly," and " On Vegetable Astringents.") 



I am particularly anxious to draw to these masterly researches 

 of Dr. Bostock, the attention they so well deserve, and have yet 

 to receive. Hitherto it appears they have been almost unknown, 

 or overlooked, as not containing facts so closely connected with, 

 the present subject. 



In Sir H. Davy's " Agricultural Chemistry," published 1813, 

 the process recommended in 1803 is repeated with little variation, 

 and a table is given of the quantities of tan in various barks, 

 estimated by the jelly test. This table is copied into the last 

 edition of Brande's *' Manual of Chemistry," without any expres- 

 sion of a doubt of its correctness in principle, and also into the last 

 edition of Henry's " Elements of Chemistry," in which we find 

 stated (vol. ii. p. 358): "In general, however. Dr. Bostock has been 

 led to conclude that the compound formed by the union of jelly 

 and tan consists on an average of somewhat less than two parts 

 of tan to three of gelatine ;" whereas Dr. B.'s last paper (above- 

 mentioned) leaves us no hope of any data to ground our calcula-; 

 tions on. 



In the Herculean task which an editor of a systematic work 

 on chemistry necessarily undertakes, it is a moral impossibility 

 that he can find time to consider the bearing which all the expe- 

 rimental facts, scattered through our numerous scientific jour- 

 nals, have on received opinions and theories. Such omissions 

 are continually occurring in similar elementary works on other 

 sciences, in the hands of most diligent and faithful compilers. 

 For my own part I am so satisfied of the proper feeling enter- 

 tained on such points by the gentlemen at the head of the science 

 to which I have the honour to be attached, that having once 

 called their attention to the matter, I will leave its adjustment 

 entirely to them. 



In endeavouring to strike out an unexceptionable process for 

 the use of tanners, and complete this test in the spirit oi utility 

 in which Sir H. Davy had first conceived it, I found it necessary 

 to take a different path from that pursued by Proust and Troms- 

 dorff*, who endeavoured by the action of reagents to deprive tan 



