Intelligence and Miscellaneous Articles. 235 



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radius of the curvature of the lens in question. In like manner the 

 curvature of the other surface of the lens may be discovered. I have 

 found this method useful in ascertaining the radii of the surfaces 

 of lenses forming the eye piece of telescopes and microscopes, to 

 which, as they are of short foci, it is more particularly applicable. 

 I have mentioned sealing-wax, as being the material most easily ob- 

 tained for the experiment ; but I have found fusible metal to be far 

 preferable. In employing this, however, there is some chance of 

 injuring the lens, in consequence of the greater degree of heat em- 

 ployed. The mode of using the metal (as in the French method of 

 forming medallions) is to melt it with as little heat as possible, and 

 pour it upon a piece of paper ; then, as soon as, in cooling, it begins 

 to get pasty at the edges, to stamp the lens upon it : a tolerably 

 brilliant mirror will be the result. 



Will you allow me, as a postscript to this communication, to 

 make the following inquiries ? In the Edinburgh Philosophical 

 Journal for April 1823, it is stated that the late Sir William Her- 

 schel had composed a work on the subject of grinding and polish- 

 ing specula for telescopes, and that it was his intention to publish 

 it. Was his intention ever carried into effect ? If not, may the 

 scientific world still hope to possess the work of one so eminently qua- 

 lified to instruct others in a difficult, and, I believe, little understood 

 mechanical operation ? In connexion with the same subject, Sir 

 John F.W. Herschel mentions, in his letter from the Cape, contained 

 in 3'our Number for June, that he had taken out with him " a com- 

 plete polishing apparatus with the efficacy of which he was perfectly 

 satisfied." Now, as the ordinary process of polishing, and giving a 

 correct figure to, the specula of telescopes is most tedious and un- 

 certain, can any of your correspondents furnish a description of so 

 valuable an apparatus, or could Sir John Herschel (when unoccu- 

 pied by more important engagements) be prevailed upon to make 

 known the construction of the engine to the public ? 



Collumpton, April 18, 1835. 



ARSENIC IN ENGLISH SULPHURIC ACID. 



Vogel of Munchen infers from his experiments on sulphuric 

 acid: 



1. That the Nordhausen acid, prepared from the sulphate of 

 iron, contains no arsenic ; the precipitate with sulphuretted hydro- 

 gen being pure sulphur. 



2. Concentrated English sulphuric acid prepared in leaden 

 chambers contains arsenic, and the precipitate produced in it by a 

 current of sulphuretted hydrogen consists of sulphur and orpiment. 



3. No precipitate of sulphur takes place, in consequence of a 

 current of sulphuretted hydrogen being passed through English 

 sulphuric acid, diluted with from four to six parts of water; the 

 precipitate consisting of an orange yellow powder, or orpiment. 



4. Rectified English sulphuric acid contains no arsenic; this 

 substance remaining in the residue. The rectified acid diluted 

 with water is not rendered muddy by sulphuretted hydrogen. The 



2H2 



