THE TECHNOLOGIST. [Nov. 1, 1864. 



158 ON CHEMISTRY APPLIED TO THE ARTS. 



sulphuric acid by the action of the oxygen of the sesquioxide of 

 manganese, and the protoxide thus produced unites with the sulphuric 

 acid, which is soluble in water. The skins thus bleached when dressed 

 are ready for market. 



Gilding of Leather. — The usual mode of ornamenting leather with 

 gold is to apply, in such parts as are desired, a thick solution of albumen 

 covering those parts with gold-leaf, and applying a hot iron, when the 

 albumen is coagulated and fixes the gold. This plan is objectionable 

 when the goods are intended for shipment, and the following method, 

 lately proposed, is far preferable ; on the parts required to be gilt, a 

 mixture, composed of ^five parts of copal and one of mastic, is spread ; 

 a gentle heat is applied, and when the resins are melted the gold-leaf is 

 spread upon them. 



Parchment. — There are two distinct qualities of this valuable mate- 

 rial, which has been used from time immemorial as a means of preserving 

 records. The best quality is prepared from young lamb, kid, and goat 

 skins, and the second quality from calf, wolf, ass, and sheep skins. To 

 make parchment, the following is the process : — The skins are stretched 

 on strong rectangular frames, limed, unhaired, fleshed very carefully, and 

 rubbed with pumice-stone until they have acquired the proper thickness. 

 They are then dried very carefully in the shade. 



Dialysis. — Mr. Thomas Graham, Master of the Mint, has lately 

 drawn the attention of the scientific world to a most remarkable 

 property possessed by organic membranes, of separating when in solution 

 crystallisable bodies from those which are not so. The former he names 

 crystalloids, and the latter colloids. For instance, if a solution of sugar 

 (crystalloid) is mixed with one of gum (colloid) and placed in the vessel, 

 the bottom of which consists of a septum of animal or vegetable parch- 

 ment, the crystalloid sugar will pass through the membrane into the 

 surrounding water, whilst the colloid gum will remain in the vessel. 

 Again, if solutions of iodide of potassium and albumen be mixed 

 together, the iodide of potassium will diffuse itself through the mem- 

 brane, which the albumen will not do. Also, if to an alkaline solution 

 of silicate of soda weak hydrochloric acid be cautiously added, chloride 

 of sodium will be produced, and silica will remain in solution ; and if 

 such a solution be placed in the dialyser, the chloride of sodium (the 

 crystalloid) will diffuse itself through the membrane while the silica 

 (the colloid) will remain behind. It is impossible to calculate the 

 immense service which the discovery of these facts by Mr. Graham will 

 render to physiology, toxicology, and to manufactures, as, in fact, every 

 day new applications of it are being made in these various departments 

 of human research. Thus, to give an example which has special refer- 

 ence to the lectures, I have lately seen it proposed by Mr. A. Whitlaw 

 to place salted meat in large dialysers, when it is stated that the salt 

 only will be removed, leaving all the nutritive properties of the meat 

 undiminished. Mr. Whitlaw also proposes to dialyse the brine in which 



