PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 667 



glass is taken from the window, and plunged for several daj-s in a 

 weak^ solution of carbonate of soda; it is next washed in clean 

 water and afterwards dipped for some days in a solution of hydro- 

 chloric acid sp. gr. 1,080. In this way the colors become as beau- 

 tiful as when they were first transmitted. 



Tough Soap-Bubbles. 

 Those who have experimented with ephemeral refractin 

 spheres blown from common soap and water, will be glad to know 

 that bubbles of extraordinary size and strength can be formed 

 .from a mixture of oleate of soda and glycerine. They may be 

 kept on glass or under bell-jars for 24 honrs. When dropped 

 upon the floor they rebound like a ball. If carefully cut open 

 with a pair of scissors, wet with the sohition, smaller spheres 

 may be blown inside. Under the microscope the movements and 

 iridescence of a small bubble present many beautiful changes. 



Perchlokate of Thallium. 

 Prof. H. E. Roscoc regards this salt as isomorpnous with the 

 potassium and ammonium perchlorates. The angles observed by 

 Mitscherlich in case of the latter, agree exactly with those of the 

 crystalliue thallium perchlorate. The isomorphism of ammonium 

 and thallium sulphates having been previously determined, these 

 resemblances in structure, as well the fact that thallium alum has 

 been formed, furnish strong grounds for placing thallium in the 

 group of alkaline metals. Although it is devoid of prominent 

 alkaline characteristics, doubtless the similarity in the structure of 

 the salts mentioned arises from the fact that it, like all the alkaline 

 metals, is monatomic. Odling, however, regards thallium as a 

 triatomic metal or a triad, like antimony and bismuth. 



Blasting with Coal. 

 M. Lapparent, of France, has invented a movable furnace for 

 disintegrating very hard rock by means of heat, in cases where it 

 is not practicable to use gunpowder. The furnace, on wheels, is 

 placed directly in front of the rock to be removed, and after the 

 coal is well ignited, a jet of compressed air and drops of water arc 

 forced into the fire, and the whole of the products of combustion 

 are brought to bear against the rock. In a few seconds the splin- 

 ters and fragments begin to fly in all directions above and l)elow 

 the furnace. When the rock is very hard, the attendant wears a 



