Mr. J. Wilson's Researches into the Alum Manufacture. 413 



If the conduction on the surfaces of insulators did not make the 

 comparison a very precarious one, it would be interesting, with 

 a view to their classification, to compare the smallest dimensions 

 at which discs of different non-conducting substances percep- 

 tibly diminish the influence of a certain body. Glass, mica, and 

 shell-lac conduct on the surface better than in the interior, 

 gutta-percha worse ; and even when the greatest care is used in 

 the treatment of the plates, this conduction cannot be retained 

 sufficiently constant as to prevent the same plate at different 

 times from exercising a considerably different action. The ex- 

 amination of a plate, by using it as an interposed plate, is much 

 more delicate than its direct examination by means of an electro- 

 scope ; and differences in superficial conduction can be detected 

 thereby, which, by the latter method, remain completely hidden. 



LVIII. Researches into the Alum Manufacture. By John Wilson, 

 Esq., Jun., Student in the Laboratory of St. Thomas's Hospital 

 College, London*. 



THE history of alum -making is of considerable antiquity. 

 Dioscorides and Pliny describe a substance, alumen, which 

 however does not seem to have had much, if any, resemblance to 

 the alum of the present day. Pliny chiefly speaks of two kinds 

 of alumen, the liquid and the solid (liquidum spissumque) ; the 

 liquid should be limpid and milk-white, and its characteristic is 

 its striking a black colour with the juice of the pomegranate ; and 

 he informs us that it is very astringent, and is efficacious when 

 mingled with honey as a cure for ulcers of the mouth. From 

 this character of giving a black with pomegranate juice, it appears 

 to have contained iron as one of its ingredients, which is not 

 unlikely if the salt was native. 



There was also a species of alumen called by the Greeks 

 schistos, which split up into capillary filaments and liquefied 

 when heated ; this substance was prepared for use by heating it 

 till it ceased to turn fluid, or in other words, by expelling its 

 water of crystallization. This description agrees with the cha- 

 racters of some alum shales which become decomposed by expo- 

 sure and split up, while hair salts shoot from the cavities. 

 Dioscorides states that there are many kinds of alumen, but 

 that most of its varieties may be found in Egypt. The best for 

 medical purposes is very white, astringent, granular, and free 

 from hard concretions, and splits up into capillary crystals. 



The date of the discovery of our alum is unknown ; previous 

 to the middle of the fifteenth century, all the alum used in 

 * Communicated by Dr. Robert Dundas Thomson, F.R.S. 



