383 



author found, by forming galvanic circles with two different metals 

 and an interposed acid, that when he used different kinds of acid, 

 or varied the degree of their dilution, the electro- magnetic effects, as 

 measured by a delicate galvanometer, bear no sort of relation to the 

 conducting power of the fluid, as is assumed in the Voltaic hypothesis. 

 He deduces the same conclusion from experiments made with an ap- 

 paratus by which the fluid is confined in a rectangular box, divided 

 by a membranous diaphragm into two compartments, so as to allow 

 of the addition of an acid to the fluid contained in one of the com- 

 partments, and thereby limiting its action to one of the metallic sur- 

 faces. By means of another contrivance, the author ascertained that 

 of two different metals, the one which, when acted upon by an acid, 

 combines with the greatest quantity of oxygen, as measured by the 

 volume of hydrogen disengaged, is always positive with respect to 

 the other metal. Even two pieces of the same metal, differing in 

 hardness, will be acted upon by the same acid in different degrees, 

 and may thus be brought into different states of electricity. In ge- 

 neral it is the harder of the two pieces of metal which becomes po- 

 sitive ; but with steel the reverse obtains. It would appear, how- 

 ever, that with the same pairs of metallic discs, the direction of the 

 electric current is determined by the nature of the acid employed : 

 thus nitrous acid, acting upon zinc, copper, or iron, gives rise to a 

 current in a direction opposite to the current which is produced by 

 the sulphuric, nitric, or muriatic acids. Variations in the tempera- 

 ture of the metals will also occasion diversities in the results, not 

 hitherto satisfactorily explained on any theory. From one experi- 

 ment the author is led to infer that an acid is capable of combining 

 with a pure metal, without the latter being previously reduced to the 

 state of an oxide. 



The Bakerian Lecture. On the Manufacture of Glass for Optical 

 Purposes. By Michael Faraday, Esq. F.R.S. #c. Read Novem- 

 ber 19, December 3 and 10, 1829. [Phil. Trans. 1830, p. 1.] 



As an introduction to his paper, the author gives a short account 

 of the circumstances which have led to the present inquiry. He states 

 the difficulties that exist in procuring glass sufficiently homogeneous 

 to answer the purposes of the optician, and adverts to the efforts made 

 by Guinand and by Fraunhofer to overcome them. As the art was 

 still imperfectly known in this country, the President of the Royal 

 Society in the year 1824 suggested the appointment of a committee, 

 whose labours were facilitated by the Government removing the re- 

 strictions imposed by the excise laws to experiments on glass, and 

 also undertaking to bear all the expenses of the inquiry, as long as 

 it held out a reasonable expectation of ultimate success. An expe- 

 rimental glass-house was at first erected on the premises of Messrs. 

 Pellatt and Green, at the Falcon Glass-works ; but Mr. Faraday 

 being unable to conduct them at that distance from his own resi- 

 dence, the President and Council of the Royal Society obtained leave 



