4 Professor Dewar [Jan. 20, 



of the absolute temperature. Many further experiments must, 

 however, be made before the real law of radiation at low temperatures 

 can be strictly defined. 



To produce exceedingly high vacua in vessels used for such pur- 

 poses as the collection and storage of liquid air, a mercurial vacuum 

 made in the following manner has been found highly satisfactory. 

 Take, as an illustration, a glass vessel shaped like Fig. 3, and 

 after placing in it a quantity of mercury and connecting the pipe H 

 with a good working air-pump, place in an oil or air-bath heated 

 above 200° C, and distil off a good quantity of the mercury. While 

 the distillation is taking place the tube is sealed off at the point A, 

 and the bulb instantly removed from the heated bath to such an 

 extent as to allow condensation to take place in the small chamber 

 marked B W in the figure. As the air-pump can maintain a vacuum 

 of ten millimetres, no difficulty arises in sealing the glass tube during 

 the continuance of the distillation. In somo cases the mercury in the 

 vessel has been heated up to near its boiling point in air, and then 

 the air-pump started, causing thereby an almost explosive burst of 

 mercurial vapour which very effectually carries 

 Fig. 3. all the air out of the vessel. After cooling, the 



vessel is removed from the bath, and the excess 

 of mercury brought into tho small bulb B W, 

 care having been taken to remove any small 

 globules of mercury, which adhere with great 

 tenacity to the surface of the glass, by heating 

 K while the part B W is kept cool. In this way 

 the vessel K is filled with nothing but mercurial 

 vapour, the pressure of which depends solely 

 on the temperature of the liquid mercury contained in the small 

 enlargement, and as this can, if necessary, be cooled to —190° C. by 

 immersing it in liquid air, we have the means of creating in K a 

 vacuum of inconceivable tenuity. It is sufficient for the production of 

 very good vacua to cool the mercury in B W to — 80° C. (using solid 

 carbonic acid as the cooling agent), and while in this condition to seal 

 off the bulb containing all the condensed mercury, so that K is left 

 full of saturated vapour at — 80° C. When very high exhaustions 

 are required it is better not to seal off the mercury bulb, as the 

 glass is apt to give off some kind of vapour. A similar mode of 

 proceeding is adopted when other shaped vessels have to be highly 

 exhausted, and no difficulty has arisen in operating with vessels 

 having the capacity of more than a litre. 



The perfection of the vacuum, assuming that nothing remains but 

 molecules of mercury in the form of vapour, depends upon the tem- 

 perature to which the subsidiary bulb is cooled. The well-known 

 law which expresses approximately the relation between temperature 

 and pressure in the case of saturated vapours, must be assumed to be 

 applicable to mercury vapour at temperatures where direct measure- 

 ment becomes impossible. Having calculated the constants of a 



