Vol. XXII. No. 9.] 



POPULAR SOIEl^CE NEWS. 



183 



j^ratttcal Cfjcmietrp anU tbt arte. 



TANNING. 



The art of tanning is a very ancient one. It 

 is rather singular that mankind should have 

 discovered the peculiar action of the constitu- 

 ents of bark upon the skin of animals at so 

 early a period, and it is hard to guess what 

 fortunate accident first showed the superiority 

 of leather over the natural hide. 



The change of raw hide into leather is both 

 a physical and a chemical one. The tannin, 

 or tannic acid, which is present in the bark of 

 certain trees, unites chemically with the gela- 

 tine in the skin, forming a tough, insoluble com- 

 pound. A decoction ol nutgalls poured into a 

 solution of gelatine in warm water will give an 

 abundant precipitate of this substance. 



A similar compound is formed when milk is 

 poured into a cup of tea, the tannin present in 

 the infusion uniting with certain nitrogenous 

 constituents of the milk. This change in the 

 chemical nature of the gelatine of the raw 

 hide also changes the physical character, 

 renders it soft and pliable, and prevents the 

 fibres from agglutinating together. 



The hides undergo several preparatory pro- 

 cesses before they are ready to be tanned. They 

 are first soaked for several daj-s in water con- 

 taining some carbolic acid to render them soft, 

 while the acid prevents putrefaction in the 

 subsequent processes. They are next placed 

 in vats filled with a mixture of slaked lime 

 and water. This removes the fat, and loosens 

 the hair, which is afterwards shaved off with a 

 blunt knife. To do this with greater ease, the 

 hides are placed upon a cylindrical piece of 

 wood inclined at an angle towards the work- 

 man, and called the "tanner's wooden leg." 

 It occupies the same position, and at a little 

 distance resembles an artificial leg held out 

 before him by the tanner. 



After carefuU}- removing the hair, flesh, and 

 superfluous portions of the hides, they are 

 placed in what is known as the " sour-bath." 

 This varies in composition, but is essentially a 

 liquid which has fermented or soured, and be- 

 come charged with weak acid. European tan- 

 ners use for this purpose a liquid made by boil- 

 ing waste malt, bran, etc., in water. This bath 

 removes the lime, and causes the hides to swell. 



The bark of the oak-tree is preferred for 

 tanning purposes ; but that of the hemlock is 

 cheaper, and is extensively used. The leaves 

 and stems of the sumach-tree are also used. 



The tanning takes place in large tanks or 

 vats sunk into the ground. The hides are 

 placed in them along with chips of ground 

 bark, and the vats filled with an extract made 

 by boiling the ground bark in water. Here 

 they remain for several weeks or months, the 

 bark and extract being renewed from time to 

 time. Formerly a year or more was allowed 

 for the conversion of the hides into leather ; 

 but at present a much shorter time is con- 

 sidered sufficient, although the quality of the 

 leather is apparently the worse for the en- 

 hanced speed of tl* process. After the tan- 

 uing process is complete, the leather is taken 



from the vats, washed and dried, and is ready 

 for the oiling and dressing of the currier. 



There are other methods of tanning, which 

 are used for the lighter and finer kinds of 

 leather, although the chemical change is quite 

 different from that produced by tannic acid. 

 Chief among these is the process of "tawing," 

 in-which alum is used. The salts of ahimin- 

 ium seem to unite with the fibres of the skin, 

 forming a leather closely resembling that pro- 

 duced by the older method. Hides can also 

 be converted into leather bj' thoroughly satu- 

 rating them with oil. A part of the oil combines 

 directly with the skin, while the excess is after- 

 wards removed. Wash-leather, or " chamois- 

 skin," is prepared in this manner. The pecul- 

 iar odor of Russia leather is due to an oil 

 distilled from birch wood with which the 

 leather is treated after being tanned with bark 

 in the usual way. 



Recipes for quickly tanning the skins of 

 small animals are often ofl^ered for sale by 

 peddlers and peripatetic adventurers. They 

 are usually simply variations of the alum pro- 

 cess, or else some worthless mixture of acids 

 which temporarily renders the skin soft, only to 

 subsequently cause its complete destruction. 



AN ELECTRICAL-CURRENT METER. 



A NEW method of measuring a current of 

 electricity has been devised by Professor 

 George Forbes. The accurate measurement 

 of the amount of electricitj' supplied for light- 

 ing or other purposes has been an important 

 and diffleult problem. The apparatus of 

 Professor Forbes is extremely simple in prin- 

 ciple, although there were many mechanical 

 difficulties necessary to be overcome before a 

 practical meter could be produced. 



The action of the meter depends upon the 

 heating power of the electric current. Before 

 passing into the lamp or motor, the whole or 

 part of the current passes through a flat, 

 horizontal coil of wire, above which is sus- 

 pended a little wheel provided with inclined 

 vanes like a miniature windmill. As the coil 

 becomes heated by the current, the hot air 

 rises, and sets the wheel in motion ; and the 

 number of revolutions, which are proportional 

 to the amount of electricity used, are regis- 

 tered by gear-wheels connected with it, the 

 same as in an ordinary gas-meter. Variations 

 in the strength of the current heat the coils 

 more or less, and cause the registering-wheel 

 to turn faster or slower. Unlike all other 

 meters, it is adapted to measuring alternating 

 currents of electricity, in which the direction 

 is reversed many times a second, as well as 

 the currents which flow continuously in the 

 same direction. 



The instrument is certainly a most ingen- 

 ious one, and, if found to be practicable for 

 general use, will be an important addition to 

 the list of modern electrical inventions. 



A NEW LECTURE APPARATUS FOR DE- 

 MONSTRATION OF REFLECTION AND RE- 

 FRACTION. 



The apparatus of which a brief description is 

 here offered, is so simple, and in every particular so 

 much more convenient than any other designed for 

 the same purpose, that the writer deems it worth 

 bringing to the attention of his fellow-teachers in 

 physics. 



The refracting medium is a hemi-cylinder of 

 crown glass, carefully polished, and mounted so as 

 to turn on its axis horizontally. The radius is 

 2.5 centimeters, tlie length 5 centimeters. The axis 

 passes through the centre of a circle of white card- 

 board, whose radius may be 20 centimeters, or 30 

 centimeters. Each of its quadrants is graduated 

 from 0° to 90°, as shown in the diagram, the 

 diameter connecting the two zero.points being 

 perpendicular to the face of the hemi-cylinder. 

 From a horizontal slit in front of the lantern, a 

 beam is sent through the middle of the glass, and 

 focussed on the zero-point at the farther edge of 

 the card-board, whose plane has been slightly in- 

 clined so that the path of the beam is sharply de- 

 fined upon it. The incident, reflected, and trans- 

 mitted beams are in the same line, the angle of 

 incidence being zero. 



The hemi-cylinder is now rotated through any 



The art of paper-making has reached a point 

 where a growing tree may be cut down, made into 

 paper, and turned out as a newspaper, all within 

 thirty-six hours. 



desired angle, for example 50°, as shown in Fig. 1. 

 The card-board moves with it. The room should 

 be but slightly darkened, so that there may be no 

 difficulty in reading the graduation on the circle. 

 Part of the beam is reflected, and part refracted. 

 Both are plainly seen, and the angles of reflection 

 and refraction are measured. Varying the angle 

 of incidence from 0° to 90°, one readily ob.serves 

 that the ratio of the reflected light to the refracted 

 light decidedly increases. Turning still further, 

 the beam from the lantern strikes normally on the 

 curved surface, and is totally reflected within the 

 glass at its plane face (Fig. 2). Rotating still fur- 

 ther, the re-appearance of the refracted beam an- 

 nounces the critical angle, which is read upon the 

 circle. 



In the common form of apparatus where a beam 

 is deflected by a mirror, then sent through smoky 

 air and cloudy water, new adjustments are neces- 

 sary for every variation of the angle of incidence, 

 involving some trouble and loss of time. With the 

 hemi-cylinder, but one easy adjustment is needed 

 for all. The higher refractive index of glass is an 

 additional advantage, aside from the superior defi- 

 nition. The expense is scarcely more than that of 

 having the hemi-cylinder made by a practical 

 worker in glass. A small silvered mirror is substi- 

 tuted for the hemi-cylinder if the law of reflection 

 alone is to be exhibited. — W. LeConte Stevens, 

 in American Journal of Science. 



AN EASY METHOD OF FINDING THE 

 SPECIFIC GRAVITY OF LIQUIDS. 



A NEW application of an old rule has suggested 

 a method of finding the specific gravity of liquids 

 which I have never seen mentioned, and which, 

 from its simplicity and great ease of application, 

 seems worthy of publication. By means of it the 



