1869.] Phijsics. 317" 



The oxyhydrogen light, in which the burning jet of mixed 

 gases is allowed to impinge on a piece of Zirconia instead of Lime, 

 is gradually being introduced in Paris. The advantages of em- 

 ploying Zirconia are, that of all the known earthy oxides it is the 

 only one which remains entirely unaltered when submitted to the 

 action of the blowpipe fed by oxygen and hydrogen. Zirconia is 

 also, of all the earthy oxides, the one which, when introduced into 

 an oxyhydrogen flame, develops the most intense and the most 

 fixed lisfht. 



All persons interested in optical researches will be glad to hear 

 that Messrs. Chance are now making optical glass of a density of 

 4 • 4. Glass of this density has never before been prepared com- 

 mercially in England, although we believe it has long been made 

 and used in France. 



Heat. — M. Le Eoux has made some experiments with the 

 vapour of sodium, and examined its capability or incapabihty of 

 passing through rock-salt. Two crucibles of rock-salt were pre- 

 pared, a thin plate of the same substance placed between them, and 

 in one of the cavities sodium was placed. 



Notwithstanding a bright red heat maintained for several hours, 

 the piece which was not in direct contact with the sodiiun vapour 

 remained completely unaltered, even where it had been in contact 

 with the plate already completely penetrated. Chloride of sodium 

 is not attacked by the vapour of sodium, but soda corrodes it ener- 

 getically. A very small quantity of soda suffices to hermetically 

 seal two surfaces of rock-salt, sodium preserving its lustre for several 

 months in a crucible of this kind. Potassium vapour does not attack 

 its chloride, but it covers the chloride with a bright blue substance, 

 in which, possibly, chemists recognize the suboxide of potassium. 



Mr. W. P. Dexter has described a new gas-lamp for heating 

 crucibles, &c. The ordinary Bunsen burner is known to act 

 upon the surface of platinum vessels brought into contact with the 

 inner line of the flame ; the metal loses its polish, becoming super- 

 ficially porous and spongy, and requires the use of the burnisher 

 to bring it back to its original state. This alteration of the surface 

 is attended with a change of weight, and Mr. Dexter has conse- 

 quently devised the following arrangement : — He removes the air- 

 tube of a common Bunsen lamp, and puts in its place a somewhat 

 longer one of glass or iron, of about 12 millimetres internal dia- 

 meter. The gas-jet has a single circular aperture, and should be 

 in proper proportion to the diameter of the tube, which may be held 

 in any of the ordinary clamped supports. The tube being raised 

 sufficiently above the jet to allow free entrance of air, and a full 

 stream of gas let on, a " roaring " flame is produced, of which the 



