352 



want of opportunity, he was obliged to abandon the inquiry before 

 arriving at any determinate results : though as far as they went, they 

 appeared to confirm the conclusion that the effects were dependent 

 on the degree of light, and not on that of the heat. The red rays, 

 however, appeared to have a greater effect in diminishing the ter- 

 minal arc than the blue. In order to determine the single effect of 

 temperature, independently of light, the needles were vibrated in 

 close vessels surrounded with water of different temperatures ; the 

 results showed that the terminal arc was increased in air of higher 

 temperatures, which is the reverse of what takes place from the di- 

 rect influence of the solar rays ; and that this effect, instead of being 

 different in the magnetized and in the other needles, was nearly the 

 same in all, of whatever materials they consisted, and whether mag- 

 netized or not. The author next endeavoured to ascertain the effects 

 produced on the axes of vibration by the action of a common fire ; 

 these, though much less in degree, he found to be similar in kind to 

 those of the sun. 



The Bakerian Lecture. On a Method of rendering Platina malleable. 

 % William Hyde Wollaston, M.D. F.R.S. #c. Read November 20, 

 1828. [Phil. Trans. 1829, p. 1.] 



In this paper the author details the processes which, from long 

 experience in the treatment of platina, he regards as the most effec- 

 tual for rendering that metal perfectly malleable. When it is purified 

 by solution in aqua regia, and precipitation with sal-ammoniac, suf- 

 ficient care is seldom taken to avoid dissolving the iridium contained 

 in the ore by due dilution of the solvent. The author states the exact 

 degree of dilution requisite for this purpose, and the exact proportions 

 in which the two acids are to be used. The digestion should be con- 

 tinued for three or four days, with a heat which ought gradually to 

 be raised ; and the fine pulverulent ore of iridium allowed to subside 

 completely before the sal-ammoniac is added. The yellow precipi- 

 tate thus obtained, after being well washed and pressed, must be 

 heated with the utmost caution, so as to expel the sal-ammoniac, 

 but at the same time to produce as little cohesion as possible among 

 the particles of platina. It is then to be reduced to powder, first by 

 rubbing between the hands, and next by grinding the coarser parts 

 in a wooden mortar with a wooden pestle ; because the friction of any 

 harder substance would, by producing burnished surfaces, render 

 them incapable of being welded together by heat. The whole is then 

 to be well washed in clean water. 



In this process the mechanical diffusion through water is made to 

 answer the same purposes as liquefaction by heat in the case of the 

 other metals ; the earthy impurities being carried to the surface by 

 their superior lightness, and the effect of fluxes being accomplished 

 by the solvent powers of water. 



The grey precipitate of platina being thus obtained in the form of 

 an uniform mud or pulp, is now ready for casting, which is effected 



