432 LEATHER EOT A CHEMICAL COMPOVNI). 



coil with its Foucault break in the best order, a batter}' of Leyden-jars care- 

 fully proportioned to the Plilcker's tube in use, a heliostat, which of course 

 involves clear sunshine, an optical train of slits, prisms, lenses, and camera 

 well focused, and, in addition to all this, a photographic laboratory in such 

 complete condition that wet, sensitive plates can be prei)ared which will 

 bear an exposure of fifteen minutes and a prolonged development. It has 

 been difficult to keep the Pliickei^s tubes in order ; often before the first 

 exposure of a tube was over, the tube was ruined by the strong Leyden 

 sparks. Moreover, ito procure tubes of known contents is troublesome. For 

 example, ray hydrogen-tubes gave a spectrum photograph of fifteen lines, of 

 which only three belonged to hydrogen. In order to be sure that none of 

 these were neAv hydrogen-lines, it was necessary' to try tubes of various 

 makers, to prepare pure hydrogen and employ that, to examine the spec- 

 trum of water, and finally to resort to comparison with the sun." 



In regard to the significance of the inquiry in relation to spectroscopic 

 study, Dr, Draper remarks : 



•' We must, therefore, change our theory of the solar spectrum, and no 

 longer regard it merely as a continuous spectrum with certain rays ab- 

 sorbed by a layer of ignited metallic vaj)ors, but as having also bright lines 

 and bands superposed on the background of continuous spectrum. Such a 

 conception not only opens the waj^ to the discovery of others of the non- 

 metals, sulphur, phosphorus, selenium, chlorine, bromine, iodine, fluorine; 

 carbon, etc., but also may account for some of the so-called dark lines, by 

 regarding them as intervals between bright lines." --Poj9?/iar Science ATonthly. 



LEATHER NOT A CHEMICAL COMPOUND. 



Chemistry, though it is not exceptional in that respect, is a science in 

 which we are continually unlearning as well as learning. It was formerly 

 believed, and the text-books still assert, that leather is a true chemical com- 

 bination formed by the hide and the astringent matter. The researches of 

 Knapp have thrown new light upon this question, proving that leather can 

 not possibly be a chemical compound. He has succeeded in making leather 

 without any tanning matter by merely driving the water out of the pores 

 of the hide by means of chloride of calcium and anhydrous ether, and he 

 has then reconverted this leather into its original state of hide by leaving it to 

 steep in water. The experiments of Knapp show that in tanning, the 

 special agent are not absorbed by the hide in an invariable quantity, but 

 that the proportions depend on the degree of concentration and on the na- 

 ture of the solvent. To penetrate into the hide, to enfold the fibres, to 

 cover them with a precipitate by surface attraction — this is the only part 

 played by the tanning principles. Owing to their presence, the fibres dur- 

 ing the drying of the hide do not form a horny mass, but remain supple and 

 flexible. Leather is a really mechanical mixture, and tanning is only a spe- 

 cial case ot dyeing — Boston Journal of Chemistry. 



