462 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



Rubidium and caesium yield alums with alumina and sulphuric 

 a«id, and the difference in the solubility of these salts affords a 

 method for their separation. 



The ratio of solubility of the alums is: 



Potash _ 22 



Cesium , _. 4 



Eubidium _ 1 



A very interesting discovery was made by Pisani, of Paris. A 

 gentleman, who had a collection of minerals, sent a number of 

 them to Pisani to have them properly named and described. They 

 were all of them examined before the spectroscope. One of them 

 known to mineralogists under the name of Pollux, was found to 

 give the blue cesium lines with remarkable definition. It was 

 subjected to a careful analysis, and found to contain the extraor- 

 dinary amount of thirty-four per cent of the oxide of cajsium. 

 The mineral pollux occurs on the Island of Elba. It resembles 

 some varieties of fluorspar, and crystalizes in the same form. 



We would call attention to this fact, as it may eventually be 

 found in the United States. 



During the year 1865, Bunsen was occupied collecting material 

 from which to prepare metallic caesium. He intended to reduce 

 the bitartrate in the same manner as he had followed in the pre- 

 paration of metallic rubidium. 



The uses and practical application of caesium are subjects of 

 investigation. It may, hereafter, for aught we know, play a very 

 important part in the arts. 



Roofing. 



The chairman, in introducing this subject, spoke of the use of 

 pulverized slate mixed with coal tar ; also of calcined clay and 

 coal tar, and said unless very great care was taken in putting 

 these materials on, there would be danger of its cracking. The 

 evidence on the value of this kind of roofing is now conflicting — 

 some commendinfj and others condemnino: it. 



Dr. Hirsh said that if the coal tar is put on thin it will not 

 crack. In Europe they have roofing paper, not only water-proof, 

 but fire-proof, and is used in some fire-proof safes. Soluble glass 

 is mixed with it. 



Dr. Thompson, of Auburn, Cayuga county, N. Y., remarked 

 that this coal tar roofing has been much used at Auburn. On a 

 warm day the tar melts and runs off, and in a short time none of 



