406 British Association for the Advancement of Science, 



burner came within the tube the flame again retracted as before. 

 The sound ceased at the moment that the flame was extinguished. 



The pressure of gas used in most of the experiments was that of 

 the ordinary main-pipes in this city, about lj- inch of water ; that 

 of the current of air, which was produced by a good pair of double 

 bellows, was equal to the pressure of a column of (2^ inches of) 

 mercury ; but it was found that no material alteration of effect took 

 place from condensing the gas to about two atmospheres, and cau- 

 sing it to issue inflamed at that pressure, provided the pressure of 

 the current of air was likewise increased in the same ratio nearly. 



With a less powerful stream of air than was above stated, the 

 effects were imperfectly produced ; and with a much more powerful 

 one the flame was blown out. 



The temperature of the current of air heated by the flame, when 

 it retracted best, was found, at the distance of four inches from the 

 burner, to be 432° Fahrenheit, or perhaps a little higher. The com- 

 bustion of the flame in all the foregoing cases is absolutely perfect ; 

 its colour is a deep blue, and the volume of intensely heated air pro- 

 pelled is very great, so that it may be rendered very useful for 

 various purposes in the laboratory. It is not perfectly dry, but it 

 is free from dust or smoke. 



On the Volatilization of Magnesia by Heat. By Professor Daubeny. 



According to Von Buch, carbonate of magnesia must have been 

 sublimed by volcanic action, although such a phaenomenon would, 

 Dr. Daubeny conceived, be scarcely admitted by chemists as con- 

 sistent with the known properties of that earth. 



A curious fact, however, confirmatory of the truth of Von Buch's 

 opinion, occurred to Professor Daubeny in Italy. He visited a lo- 

 cality where there was an upper stratum of lava containing cavities. 

 In one of these an English gentleman, resident on the spot, discovered 

 a large quantity of carbonate of magnesia, and Professor Daubeny 

 himself observed a minute portion of the same earth coating the 

 outer surface of the lava. Here it is difficult to understand in what 

 manner this substance could collect in the cavities or upon the sur- 

 face of the rock, unless it had previously become volatilized by 

 heat*. 



(Dr. Dalton observed that there could be no doubt that carbonate 

 of magnesia might be volatilized, since Dr. Henry had informed him 

 that a quantity of this substance was always driven oft' whenever the 

 heat was carried beyond a certain point.) 



Mr. Hartop made a communication on the use of the hot air 

 blast in the manufacture of pig iron, in which he showed that the 

 saving said to be effected by the use of hot air had been overrated, 

 as a considerable portion of the alleged saving had been previously 

 effected by other improved processes. 



• [Other particulars relative to this subject are given in our last Number, 

 p. 316.— Edit.] 



