470 



experiments have been made to determine either of the points at 

 issue. The authors have sought to supply the want of data on these 

 questions hy researches on the density of steam upon a new and ori- 

 ginal method. 



The general features of this method consist in vaporizing a known 

 weight of water in a globe of about 70 cubic inches capacity, and 

 devoid of air, and observing by means of a "saturation gauge " the 

 exact temperature at which the whole of the water is converted into 

 steam. The saturation gauge, in which the novelty of the experi- 

 ment consists, is essentially a double mercury column balanced upon 

 one side by the pressure of the steam produced from the weighed 

 portion of water, and on the other by constantly saturated steam of 

 the same temperature. Hence when heat is applied the mercury 

 columns remain at the same level up to the point at which the 

 weighed portion of water is wholly vaporized ; from this point the 

 columns indicate, by a difference of level, that the steam in the globe 

 is superheating ; for superheated steam increases in pressure at a far 

 lower rate than saturated steam for equal increments of temperature. 

 By continuing the process, and carefully measuring the difference of 

 level of the columns, data are obtained for estimating the rate of 

 expansion of superheated steam. 



The apparatus for experiments at pressures of from 1 5 to 70 Ibs. 

 per square inch, consisted chiefly of a glass globe for the reception of 

 the weighed portion of water, drawn out into a tube about 32 inches 

 long. The globe was enclosed in a copper boiler, forming a steam- 

 bath by which it could be uniformly heated. The copper steam- 

 bath was prolonged downwards by a glass tube enclosing the globe 

 stem. To heat this tube uniformly with the steam-bath, an outer 

 oil-bath of blown glass was employed, heated like the copper bath by 

 gas jets. The temperatures were observed by thermometers exposed 

 naked in the steam, but corrected for pressure. The two mercury 

 columns forming the saturation gauge were formed in the globe stem, 

 and between this and the outer glass tube ; so long as the steam in 

 the glass globe continued in a state of saturation, the inner column 

 in the globe stem remained stationary, at nearly the same level as 

 that in the outer tube. But when, in raising the temperature, the 

 whole of the water in the globe had been evaporated and the steam 

 had become superheated, the pressure no longer balanced that in 



