1893.] on Liquid Atmospheric Air. 5 



vapour pressure formula from observed data at high temperatures, 

 it is easy to arrive at a value of the vapour pressure for any- 

 assumed lower temperature. Such a formula as the following, 



log. P = 15-1151 - 2-2931, log. T ^— , where P is pressure 



in millimetres of mercury and T is the absolute temperature, agrees 

 well with the experimental results. This formula gives the vaponr 

 pressure at 0° C. as 0-000 18 mill., and at - 80° C. as 0-000,000,003 

 mill., or respectively, about the sixth and the four hundred thousandth 

 of a millionth of an atmosphere. Such a high vacuum could never be 

 reached by the use of any form of mercurial air-pump. The electric 

 discharge in such vacua produces intense phosphorescence of the glass, 

 giving thereby a continuous spectrum, which makes the detection of 

 the mercury lines difficult. 



Consider for a moment the proofs that could be adduced that 

 mercury vapour, even below a millionth of an atmosphere pressure, 

 can behave like an ordinary saturated vapour. The most character- 

 istic property of a space filled with any saturated vapour is that 

 cooling to a lower temperature causes partial condensation of the 

 vapour in the form of liquid or solid. The amount of vapour in a 

 mercurial vacuum at the ordinary temperature would weigh about 

 the tenth of a milligram in the volume of a litre, and to see such an 

 amount of the metal it would require to be concentrated on a 

 small area in the form of a fine metallic film. Experiment has 

 shown that the fifth of a milligram of gold may be made to cover 

 one square centimetre of surface, so that a minute quantity of 

 metal can be observed if properly deposited. This can be easily 

 achieved in such mercurial vacua by cooling a small portion of the 

 surface of the glass to the temperature of — 180° C. by the applica- 

 tion of a pad of cotton wool saturated with liquid oxygen. In an 

 instant the vapour of mercury deposits in the form of a brilliant 

 mirror, which, on the temperature rising, becomes subdivided into a 

 mass of exceedingly minute spheres of liquid. A repetition of 

 the cooling does not bring down a new mirror, provided the first 

 area is maintained cool, but if the vessel contains excess of liquid 

 mercury any number of mirrors of mercury may be deposited in 

 succession. 



Mercury is thus proved to distil at the ordinary temperature 

 when the vapour pressure is under the millionth of an atmosphere. 

 Further, it is easy to prove in this way that the cooling of the 

 liquid mercury in "such a subsidiary vessel (which has been de- 

 scribed as a temporary part of the vacuum vessel) greatly improves 

 the vacuum. For this purpose it is sufficient to cool the said vessel 

 with some solid carbonic acid, and then to try and reproduce a 

 mirror of mercury in the way previously described. No mercury 

 mirror can be formed so long as the cold bath is maintained. If a 

 piece of blotting paper, cut into any desired shape, be moistened with 

 water, and then applied to the surface of one of the vacuum vessels 



