230 Royal Society :— 
the boiling-points of terms already known, and the examination of 
new terms, shall have laid a broader foundation for our conclusions. 
May 10.—Sir Benjamin C. Brodie, Bart., President, in the Chair. 
The Bakerian Lecture was delivered by Mr. Fairbairn, F.R.S. 
The Lecturer gave a condensed exposition of the experiments and 
results detailed in the following Paper. He also exhibited the appa- 
ratus employed, and explained the methods followed. 
«Experimental Researches to determine the Density of Steam at 
all Temperatures, and to determine the Law of Expansion of Super- 
heated Steam.” By William Fairbairn, Esq., F.R.S., and Thomas 
Tate, Esq. 
The object of these researches is to determine by direct experiment 
the law of the density and expansion of steam at all temperatures. 
Dumas determined the density of steam at 212° Fahr., but at this ~ 
temperature only. Gay-Lussac and other physicists have deduced 
the density at other temperatures by a theoretical formula true for a 
perfect gas: 
VP _459+T (1.) 
VP. 45040... 1. 
On the expansion of superheated steam, the only experiments are 
those of Mr. Siemens, which give a rate of expansion extremely high, 
and physicists have in this case also generally assumed the rate of 
expansion of a perfect gas. Experimentalists have for some time 
questioned the truth of these gaseous formule in the case of conden- 
sable vapours, and have proposed new formulee derived from the 
dynamic theory of heat ; but up to the present time no reliable direct 
experiments have been made to determine either of the pomts at 
issue. The authors have sought to supply the want of data on these 
questions by 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 15 to 70 lbs. 
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 
