236 Intelligence and Miscellaneous Articles. 



prism in connexion with a cylindrical lens. Such an arrangement 

 would be capable of showing well-defined lines if the observed me- 

 teors contained any elements which would give bright lines in an 

 ordinary spectroscope. — Monthly Notices of the Royal Astronomical 

 Society, January 11, 1867. 



EXPERIMENTS ON THE EXPANSION OP SUPERHEATED STEAM. 

 BY MM. G. A. HIRN AND A. CAZIN. 



We have proposed to ourselves to treat experimentally the follow- 

 ing question : — Steam being superheated (that is, brought to a cer- 

 tain tension, and at a pressure less than the maximum tension cor- 

 responding to this temperature), it is made to undergo a certain 

 expansion, during which it neither gains nor loses heat, it remains 

 superheated and overcomes a pressure equal at each moment to its 

 elastic force ; required to determine under these conditions the 

 final temperature and pressure. The following is the principle of 

 our method. 



The vapour being contained in a reservoir under a pressure greater 

 than that of the atmosphere, a large orifice is opened, by which a 

 jet suddenly rushes out. We may suppose a certain surface which 

 separates the vapour into two parts ; one of these parts is completely 

 expelled, the other exactly fills the reservoir at the end of the flow; 

 and its elastic force has, during the expansion, always held in equi- 

 librium the external pressure on its surface, so that this part of the 

 surface is under the given conditions. It is enough to ascertain the 

 final temperature and pressure of this part. When the flow ceases, 

 the pressure sought is equal to that of the atmosphere, and the tem- 

 perature has to be found. For this purpose it is to be observed that 

 three cases may present themselves : the vapour remaining in the re- 

 servoir may (1) be still superheated, (2) have exactly attained the 

 state of saturation, (3) be supersaturated. In the first case the 

 final pressure is less than the maximum tension corresponding to 

 the final temperature ; in the second the final pressure is equal to 

 this tension ; in the third the final pressure is the same, but part of 

 the vapour condenses, forming a visible mist in the reservoir if it is 

 provided with parallel glass plates. By varying either the initial 

 pressure or the initial temperature, so that the mist gradually dimi- 

 nishes, it ultimately disappears ; and then we have the expansion 

 under conditions very nearly corresponding to the second case. 

 Taking the pressure of the atmosphere as the maximum tension of 

 the vapour, the corresponding temperature is found from Regnault's 

 Tables, and is thus obtained with a certain approximation. Thus 

 there is no thermometer : the vapour in expanding indicates to us 

 its own temperature when it becomes cloudy, and we have only to 

 observe under what circumstances this cloudiness ceases to be pro- 

 duced. This new method is sufficiently delicate ; by its means we 

 have been enabled to resolve the question proposed with an unlooked- 

 for success. 



