THE SCIENTIFIC ASPECTS OF TANNING. 9^ 



made this subject his study for some years : the result we see 

 before us in this little machine, — the theory of its action being 

 that a piece of prepared skin will only unite with the tannic acid 

 of the liquid presented to it, rejecting everything else. This can 

 be accomplished by pressure, applied through the screw, very 

 rapidly, so that in the course of half-an-hour the piece of skin will 

 abstract all the tannic acid from the liquor, what passes through 

 the glass below j being only gallic acid, the difference between 

 the density before the operation, and the density after, giving the 

 percentage of tannin. We venture to believe that this most 

 useful machine will, within a few years, be considered an indis- 

 pensable adjunct to every tanyard, as it will enable the tanner not 

 only to analyse his new materials, but also to give him the value 

 of his liquors, and in many other ways be turned to good account. 



The discourse was illustrated by a number of specimens of 

 tanning materials and finished products. The tan-testers referred 

 lo were also shown in operation. 



Mr. W. W. Stoddart then exhibited the instrument called the 

 *' Radiometer," or Light-mill. This is so sensitive to warmth that 

 the flame of a candle or the hand placed near it makes the vanes 

 revolve rapidly. The lecturer gave a history of Mr. Crookes' 

 researches with it. 



