PHYSICAL CONSTANTS OF HYDROGENIUM. 173 
the atomic proportion Pd; H,, yields the following values for the mean coeffi- 
cient of cubical expansion :— 
Between 0° and 50°, : : 0:000058 
Between 0° and 80°, : ; 0: 000066 
The cubical expansion of palladium being 0°000033, we may say the alloy 
with hydrogen is just twice as great. If the expansion of GRAHAM’s alloy is 
assumed to be equal to the sum of the expansion of the respective volumes of 
the constituents, then the calculated result for the cubical expansion of hydro- 
gen is 0°000246, a number about one and a half times the coefficient of expan- 
sion of mercury. 
With a fused mass of palladium containing a small charge of hydrogen, the 
coefficient of expansion was found to be 0°00048, and the calculated value for 
the occluded hydrogen became then 0°00059. 
Absorption of Hydrogen at a Red Heat. 
GraHAm’s theory of the rapid passage of hydrogen through palladium at 
high temperatures, assumes at first a direct absorption of the gas, and then a 
transmission of it by a kind of “ cementation process.” That an absorption of 
hydrogen takes place with its attendant increase of volume at high temperatures 
may be shown as follows :— 
Take a strip of thin sheet palladium, four or five centimetres long, and about 
five millimetres in breadth, clamp it firmly by the end in a suitable support, so 
that the strip is free to vibrate, and insert it edgeways in the middle of a 
hydrogen flame, burning from a nozzle about a millimetre in diameter. If the 
palladium is now depressed into the inner dark cone it immediately begins to 
vibrate, producing a low musical note. 
If the flame be extinguished by stopping the current of hydrogen for an 
instant, or allowing the gas to flow, the vibration commences again, and may be 
kept up without any actual flame. 
The motion in this position in the flame is due to the absorption of hydrogen 
on the cool side next the inner cone, with its attendant increase of length, pro- 
ducing a bending of the sheet into the hot portion of the flame, where the 
hydrogen is instantly expelled from the palladium, which is forced to return to 
its original position from its natural elasticity. 
The experiments detailed in this paper have been made with palladium very 
handsomely placed at my disposal by Messrs Johnson, Matthey, and Co. of 
Hatton Garden, London, and I gladly avail myself of this opportunity of 
_ thanking them for the means of conducting this research. 
