Dr. Stenhouse's Method of preparing Benzoic Acid, 283 



one of Sir Isaac Newton's propositions in the Principia, but 

 seems entirely to have escaped the notice of Laplace, Poisson 

 and the other French mathematicians. 



Your insertion of this in your ensuing Number of the Phi- 

 losophical Magazine, will oblige 



Your obedient Servant, 

 Primrose, September 5, 1844. Thos. S. Thomson. 



XLVI, Method of preparing Benzoic Acid, 

 By Dr. Stenhouse *. 



PROCEED as in Scheele's process, by mixing finely pul- 

 ^ verized gum-benzoin very intimately with its own weight 

 of slacked lime, also in fine powder. The mixture is to be 

 repeatedly boiled with water so long as any benzoate of lime 

 is dissolved. The filtered liquid is to be evaporated to about 

 one-sixth of its bulk, and a considerable quantity of a strong 

 solution of hypochlorite of lime is to be added to it. The 

 mixture, while boiling hot, is to be decomposed by a slight 

 excess of muriatic acid, and the boiling continued till all the 

 chlorine is driven off. On the cooling of the liquid the ben- 

 zoic acid is deposited in crystals, which are but very slightly 

 coloured, the greater portion of the colouring matter having 

 been removed by the chlorine. The crystals, however, still 

 retain some resinous matter, from which impurity they may 

 be readily freed by being once or twice dissolved in a very 

 small quantity of boiling water and recrystallized. A little 

 purified animal charcoal may also be advantageously employed 

 for this purpose. 



Glasgow, September 19, 1844. 



XLVII. On the successful repetition of an Experiment in which 

 Electrical Tension is produced 'without Metallic Contact. By 

 J. P. Gassiot, Esq., F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 \70LTA'S original experiment of the separation of two discs 



' of dissimilar metals insulated by glass handles is well 

 known to the electrician : the cause of the electrical phaeno- 

 mena which are thereby produced,, has been the subject of 

 much discussion. The author of the article * Galvanism' in 

 the Ency. Metrop7 p. 210, alludes to it as follows: — " It does 

 not accord with any of the previously known laws of electrical 

 equilibrium, and appears to be totally at variance with every 



* Communicated by the Author. 



