Chemical Science, 473 



Desormes. — When steam under high-pressure is allowed to escape 

 in a roaring, violent jet, from rather a large orifice pierced in a 

 plate, and a flat disc is opposed to it at a moderate distance from 

 the aperture, the latter is powerfully forced away ; but if it be pressed 

 still more closely, as if to shut the aperture, although the vapour 

 issue on all sides, like an artificial fire-work, and presses more on 

 the disc than before, it may nevertheless be left free : not only will it 

 not be raised, but if the issue for the vapour be turned towards th6 

 earth, and the disc consequently tend to fall, as well by its own 

 weight as by the pressure of the vapour, still it will not descend. 

 The same result is obtained if the experiment be made with the 

 current of air from a strong blowing machine, belonging to an iron 

 blast furnace. 



Another fact, also very curious, though well known, is that a jet 

 of high-pressure vapour, issuing from a very hot boiler, will appear 

 as a refreshing wind, compared to another jet of vapour coming 

 from a source much less elevated in temperature, and of less than 

 one-twentieth the pressure of the former. 



From the first experiment, M. Clement concludes that the com- 

 mon safety-valves, which are real discs placed upon apertures 

 with flat edges, involve a danger inherent in their form. In fact, 

 no sooner are they raised, to permit the escape of a thin film of 

 vapour, than there immediately results the impossibility of rising 

 any further; and if the production of vapour is too considerable for 

 the small aperture which can exist, and for the strength of the 

 boiler, an explosion happens, notwithstanding that the safety-valve 

 is open. This, in fact, is what has actually occurred sometimes, 

 although it has appeared incredible. 



M. Clement attributes these effects to the vacuum which is 

 formed in the jet of vapour, in consequence of the extreme velocity 

 of its molecules, and to the conical form of the kind of tube produced 

 by the two approximated plates between which the vapour is forced 

 to pass. It dilates near the edges, in consequence of its momen- 

 tum and elasticity far below the pressure of the atmosphere, and 

 the latter acts with sufficient force on the moveable disc to resist 

 the vapour. 



The remedy to this danger may be found, without doubt, in a 

 good proportion between the orifices and their edges ; the former 

 should be large, and the latter small. The addition of a conical 

 tube to safety-valves will diminish the effect of the atmospheric 

 pressure, and of the weight with which they are charged. — Bull. 

 Univ. E. vii. p. 41. 



In the next number of the Bulletin Universelle the following 

 explanation of this phenomenon by M. Clement is given. When 

 a jet of vapour moving with great velocity does not displace a 

 moveable plate put over the orifice by which it escapes, it is 

 because a partial vacuum exists between the fixed plate and the 

 mobile disc, where a tension less than that of the atmosphere exists 

 APRIL—JUNE, 1827. 2 I 



