254 PHYSICS. 



out burning it. Ice lias been maintained for some time at temperatures 

 far above the boiling point of water, and then it sublimed away with- 

 out previous melting. The pressure necessary was found to be anything 

 below 4.6°^™ of mercury, the tension of aqueous vapor at 0° C. In the case 

 of mercuric chloride, these i)henomena may readily be produced, since 

 the critical pressure for this substance is only 420™™. In commenting 

 on these results, LeConte has called attention to their agreement with 

 well-known facts. Of course it is understood that it is the vessel which 

 is hot, not the ice; and that the temperature of the ice must be pre- 

 served at 0° by the rapid evaporation of it in the solid state. In this 

 sense, the phenomena are comparable to those of Boutigny on the 

 spheroidal state, and of Faraday on freezing mercury in a red-hot cru- 

 cible. Substances are known which sublime without melting. Such 

 bodies have their boiling points below their fusing points. By subject- 

 ing them to pressure, their boiling points may be raised above their 

 fusing points. Thus metallic arsenic, which sublimes without fusing at 

 180°, melts under pressure below a red heat. The rapid evaporation 

 prevents the body from reaching the fusing temperature. Conversely, 

 by diminishing the pressure its boiling point is lowered below that of 

 fusion. He gives a noteworthy quotation from Eegnault, published in 

 1865: "It is evident that a volatile solid body may be always subjected 

 to so slight a pressure that it will boil at a temperature inferior to 

 that at which it melts. Thus ice at the temperature of — 1° C. possesses 

 an elastic force represented by 4.27™™; in other words, it boils at a 

 temperature of — 1° C. under a pressure of 4.27™™. Ice may therefore be 

 entirely volatilized by ebullition under this feeble pressure without 

 reaching its point of fusion, which is 0° C." — {Nature, xxii, 434, 603, Sep- 

 tember, October, 1880.) 



Thomson has communicated to the British Association a method for 

 determining the critical temperature for any liquid and its vapor with- 

 out mechanism. The pure liquid is placed in a glass tube about 60<=™ 

 long, about 3 or 4*=™ of the upper portion being occui)ied by the vapor. 

 The tube is then hermetically sealed and placed in a vertical position. 

 The upper part is then warmed until the surface of separation of liquid 

 and vapor sinks below S'^™ from the top. Then warm the lower part 

 containing the liquid till the surface rises again to a convenient position. 

 Operate thus, keeping the surface of seiDaration at as nearly as possible 

 a constant position of 3<=™ from the top of the tube until the surface of 

 separation disappears. The temi^erature of the tube at the place where 

 the surface of separation was seen immediately before disappearance is 

 the critical temperature. — {Nature, xxiii, 87, November, 1880.) 



Von Burg has presented a pajier to the Vienna Academy on the action 

 of safety-valves on boilers. He shows among other things that the 

 authoritative directions given in different countries as to the size of 

 safety-valves are not at all adequate, being based on erroneous concep- 

 tions. As to the cause of the small amount of the lifting of the valve 



