238 Progress of Invention. 



on account of the metals of which it consists being unequally ex- 

 panded; and coming in contact with the metallic knob, battery 

 connection is completed. This causes a small bell, similar to that 

 used to attract the attention of the attendants in telegraph offices 

 to ring ; and thus notice is given of any undue rise of temperature 

 in the place containing the instrument. Information of fire having 

 broken out may thus be obtained, and the necessary means for its 

 extinction may, in consequence, be promptly taken. The tempera- 

 ture at which the bell will ring may be regulated by adjusting the 

 metallic knob : and the apparatus is so sensitive that a lighted 

 cigar or taper placed within a very few inches of it will cause the 

 bell to ring. 



Thermo-electric Properties of Iron. — The small quantity of 

 carbon which causes the difference between malleable and cast iron 

 produces so considerable a modification of thermo-electric properties, 

 that, while a thermo-elecfric battery of malleable iron and bronze 

 causes the needle of the galvanometer to be deflected to the left, a 

 similar battery of cast iron and bronze causes a deflection to the 

 right. The discovery of this curious fact led M. Arnould Thenard 

 to make experiments with a battery consisting of malleable and of 

 cast iron ; and the results he obtained indicated other anomalies 

 which could scarcely have been anticipated. Thus, with a battery 

 consisting of malleable iron and bronze, the deflection was found to 

 be + 23°, and with a battery of cast iron and bronze — 11° 5 ; and 

 it might be supposed that with a battery of malleable and cast iron 

 the deflection would be 23 + 11°"5 = 34 0, 5. Such, however, was 

 not found to be the case ; the deflection actually produced being 

 only 27°. The electro-motive force of the three kinds of battery was 

 in accordance with what might be expected : that of malleable iron 

 and bronze, cast iron and bronze, and malleable iron and bronze 

 being represented respectively by the numbers 691, 263, and 953 : 

 the last being very nearly the sum of the other two taken together. 



Oxygen obtained from Atmospheric Air. — Our ordinary modes 

 of obtaining oxygen for experimental and other purposes are very 

 costly or very troublesome. It may, however, be procured with 

 great facility through the medium of permanganate of soda, which, 

 as it may be used over and over again, for the purpose, will entail 

 but a trifling expense. Atmospheric air is passed over a solution 

 of the permanganate, which, after a while, becomes saturated with 

 the oxygen it has absorbed, the nitrogen having passed off. To 

 separate the oxygen from the solution, a current of vapour at a pro- 

 per temperature is substituted for the current of air, this, almost 

 without producing any change in the permanganate solution, further 

 than dilution, causes the oxygen to be evolved. Concentrating the 

 solution by heat renders it again fit for use, especially if a very 

 small quantity of the permanganate is added to it. 



Miscellaneous. — Substitute for Amalgamated Zinc in the Galvanic 

 Battery. — M. Zaliwski-Mikorski has recently informed the Aca- 

 demy of Sciences that if the zinc of a galvanic battery, instead of 

 being amalgamated, is moistened all over with oil, or even with 

 ether, or any of the liquid hydro-carbons, the duration of tho current 



