108 



DETERMINATION OF THE PER CENTAGE OP TANNIN. 



[1854. 



the painter begins he moistens the part on which he purposes 

 to worlr with distilled water, squirted on by a syringe. He 

 then paints ; if he wishes to repaint any part he moistens again. 

 As soon as the picture is finished, it is syringed over with 

 ■^ater-glass. After the wall is dry, the syringing is continued 

 as long as a wet sponge can remove any of the colour. An 

 efflorescence of carbonate of soda sometimes appears on the 

 picture soon after its completion. This may be removed, 

 either by syringing with water, or may be left to the action of 

 the atmosphere." Not to dwell on the obvious advantages 

 possessed by the stereochrome over the real fresco, (such as its 

 admitting of being retouched and its dispensing with joinings,) 

 it appears that damp and atmospheric influences, notoriously 

 destructive of real fresco, do not injure pictures executed by 

 this process. 



The following crucial experiment* was made on one of these 

 pictures. It was suspended for twelve months in the open air 

 under the principal chimney of the New Museum at Berlin ; 

 " during that time it was exposed to sunshine," " mist, snow, 

 apd rain," and nevertheless " retained its full brilliancy of 

 colour." 



The stereochrome has been adopted on a grand scale by 

 Kaulbach, in decorating the interior of the great national edifice 

 at Berlin already alluded to. These decorations are now in 

 progress, and will consist of historical pictures"}" (the dimen- 

 sions of which are 21 feet in height and 24* feet in width,) 

 single colossal figures, friezes, arabesques, chiaroscuro subjects, 

 &c. On the effect of the three finishe4 pictures, it has been 

 remarked by one whose opinion is entitled to respect, that they 

 have all the brilliancy and vigour of oil paintings, while there 

 is the absence of that dazzling confusion which new oil paint- 

 ings are apt to present, unless they are viewed in one direction 

 which the spectator has to seek for. 



Mr. A. Church has suggested that if the surface of oolitic 

 stone (such as Caen stone) is found to be protected by the 

 process already described, it might be used, as a natural into- 

 haco, to receive coloured designs, &c., for exterior decoration ; 

 the painting would then be cemented to the stone by the action 

 of the water-glass. 



Mr. Church has also executed designs of leaves on a sort of 

 terra-cotta, prepared from a variety of Way's silica rock, con- 

 sisting of 75 parts clay and 25 of soluble silica. This surface, 

 after being hardened by heat, is very well adapted for receiving 

 colours in the first instance, and for retaining them after silica- 

 tion. 



from water by persistent boiling, such water possesses an ex- 

 traordinary cohesive power, sufficient, indeed, to permit of its 

 being heated at a temperature of 275° Fahr. without boiling. 

 The adhesion of water, thus prepared, to the surface of a glass 

 tube, was shown experimentally ; the force being sufficient to 

 sustain a column of water of considerable height. The con- 

 tractile force of a soapbubble was referred to, and the lecturer 

 passed on to the exhibition of the phenomena resulting from the 

 shock of two opposing liquid veins. In this case, though the 

 forces are in opposite directions, motion is not annihilated ; but 

 the liquid, as first shown by Savart, spreads out so as to form 

 a thin transparent film, the plane of which is at right angles to 

 the direction of the jets. By varying the pressure on one side 

 or the other, or by making the jets of difierent diameters, the 

 plane film could be converted into a curved one, and some- 

 times actually caused to close, so as to form a pellucid sack. 

 A vein was caused to fall vertically upon a brass disk upwards 

 of three inches in diameter. The liquids spread laterally on all 

 sides, and formed an unbrella-shaped pellicle of great size and 

 beauty. With a disk of an inch in diameter a pellicle of at 

 least equal magnitude was formed. When a candle was placed 

 underneath the curved sheet of water, a very singular effect 

 was produced. The film above the candle was instantly dissi- 

 pated, and on moving the candle, its motion was followed by a 

 corresponding change of the aqueous surface. On turning a 

 suitable cock, so as to lessen the pressure, the curvature of the 

 film became increased, until, finally, the molecular action of the 

 water caused it to form a curve returning upon itself, and ex- 

 hibiting the appearance of a large flash. When the film com- 

 pletely embraced the vertical stem which supported the brass 

 disk, a change in the form of the liquid flash was observed. 

 The latter became elongated, and was sometimes divided into 

 two portions, one of which glided down the vertical stem and 

 was broken at its base. When the jet was projected vertically 

 upwards, large sheets were also obtained. The jet was also 

 suffered to fall into small hollow cones of various apertures, and 

 the shape of the liquid sheet received thereby some beautiful 

 modifications. The enclosed sides of the hollow cone gave the 

 liquid an ascending motion, which, combined with the action 

 of gravity, caused the film to bend, and constitute a work of 

 great beauty, — Lecture delivered at the Royid Institution. 



Phenomena Connected with the Motion of Liquids. 



BY PKOFBSSOE TVJJDALL. 



The Lecturer commenced by referring to certain phenomena 

 exhibited by liquids, and at variance with our commonly 

 received notions as to their non-cohesive character. According 

 to Donny, when the air has been, as far as possible, expelled 



* Communication from Mr. George Bunsen. 



•j- Three oftliese pictures are finished, viz. 1. The fall of Babel ; 2. Die 

 Bluthc Griechenlands (' the golden age of Grecian art and poetry '); 3. 

 The Fall of Jerusalem, (an engraving of this picture was exhibited by 

 ir^Ackerman). — Two other compositions are drawn, yiz. 4. The battle 

 of the Huns ; 5. The Crusaders' arrival before Jerusalem. — 6. This 

 subject not yet decided on. 



Determination of tlie per-centage of Tannin in Su1)Stanecs- usecl 

 for Tanning. 



BY PROFESSOR FEHLING. 



Among the various substances which precipitate tannin from solution, 

 such as gelatin, quinine, animal skin, &c., the latter has hitherto been 

 recommended as the most appropriate for determining the per-cen- 

 tage of tannin. This method of valuation has been preferred because 

 it represents in miniature the operation to which the results refer. 

 There are, however, no detailed directions for its application, and in 

 repeated trials made by the author, under a variety of conditions, he 

 has found that the tarmin is never perfectly precipitated, and that the 

 solutions soon become mouldy. Experiments with a solution of quin- 

 ine, freshly precipitated oxide of iron or alumina, did not give more 

 satisfactory results. He then tried gelatin in solution, and instead of 

 weighing the precipitate obtained, by adding an excess of gelatin, 

 preferred adopting the volumetrical method, estimating the quantity 

 of solution of gelatin of known centigrade value required to precipitate 

 the tannin. For this pm-pose it is indispensable that the precipitate 

 should separate readily, but with most kinds of tannin this is not the 

 case. The author has found it advantageous to use a dilute solu- 

 tion of gelatin, and to have the liquids quite cold. His mode of oper- 

 ating is as follows : — 



