382 Proceedings of Learned Societies. 



open air, it oxidizes freely, yielding a white oxide easily reducible 

 before the deoxidizing flame of the blow-pipe. 



The hydrated oxide is precipitated from its salts by potash and 

 ammonia, but is insoluble in excess of either of these re-agents ; 

 hence it is easily distinguished from both zinc and alumina. 



The oxide may be separated from oxide of iron, with which it is 

 associated in the zinc blende by precipitating the latter with bicar- 

 bonate of soda. The precipitated, sulphide is insoluble in alkalies. 

 The quantity of indium salts exhibited by Professor Roscoe consisted 

 of about three grains ; with these he succeeded in demonstrating its 

 properties, and exhibited the characteristic indigo spectrum in a 

 very striking manner. 



Professor Roscoe also alluded to the new discoveries made with 

 the spectroscope. Cesium and rubidium have been found to exist 

 in many articles of human consumption, such as beet-root sugar, 

 tea and coffee. Thallium has been found in many minerals in which 

 its presence was hitherto unsuspected, and to occur also in very 

 appreciable quantity in molasses, the yeast of wine, chicory, and 

 even in tobacco. 



A new and comparatively abundant source of these three rare 

 metals, cesium, rubidium, and thallium, has been discovered ; the 

 water of a spring near Frankfort leaves on evaporation a saline 

 residue which contains the three metals in appreciable quantity. 



Recently a more attentive examination of the rays emitted by 

 the sun's photosphere has been made, and it is found that it exhibits 

 no trace of potassium salts. Hence that element may be regarded 

 as being absent from the solar atmosphere. 



The spectrum of burning magnesium has been found to be par- 

 ticularly rich in chemical rays, and has consequently been used with 

 success as a photographic light. Professqr Roscoe stated that if the 

 surface of burning magnesium has an apparent magnitude equal to 

 that of the sun seen from a certain point, the chemical action effected 

 by the magnesium on that point is equal to that produced by the sun 

 when at an elevation of 9° 53'. And that at a zenith distance of 07° 

 11' the visible brightness of the sun's rays is 524*7 times that of burn- 

 ing magnesium, whilst its chemical brightness is only 3G'(5 times as 

 great as that of the burning metal ; hence the great use of the latter 

 i n photography. A thin magnesium wire produces in burning as much 

 light as Beventy-four stearine candles, and to continue this light for 

 ten hours, seventy-two grammes — about two ounces and a half — of 

 magnesium must be burnt, corresponding in effect to twenty pounds 

 of stearine candles. A magnesium lamp was exhibited, consisting 

 of a coil of magnesium wire, which w;is gradually unwound ami 

 burnt as it issued from a glass tube. Magnesium wire of a size con- 

 venient for burning is now manufactured by Mr. Sonstadt's process, 

 and sold at threepence! per foot; the combustion of ono inch of 

 wire affording sufficient light to take a positive picture with dry 

 collodion. During the lecture :i negative of Professor Faraday was 

 taken ; from this a transparent positive was printed by a few seconds 

 exposure, and exhibited on the white screen by the electric lamp. 



