236 
rOPULAR SCIENCE REVIEW. 
face force under consideration we find a power acting in perfect 
independence of gravitation — often in opposition to it ; but it is 
a caved giant, whose power is limited to the cave in which it 
dwells. 
Pursuing a series of investigations, all of them being remark- 
able examples of experimental induction, and which may be 
regarded as originating in the more simple phenomena referred 
to, Mr. Grraham was led to the discovery that certain metals 
not only absorbed some of the gases, and especially Hydrogen, 
but that they retained those metals, or as the discoverer termed 
it “ occluded ” * them. When iron or platinum or palladium in 
a state of tolerable purity — whether in the form of sponge, or 
aggregated by hammering — is heated, and allowed to cool 
slowly and completely in a hydrogen atmosphere, those metals 
are found to have absorbed many times their volume of the gas, 
and to hold it in a state of occulsion ” for any length of time ; 
until, indeed, it is dispelled by heat. It was the discovery of 
this fact, and the examination of meteoric iron, which led to 
the remarkable discovery that these meteoric masses must have 
passed through — and indeed cooled in — an atmosphere of 
hydrogen gas. Mr. Graham advanced from this point to a 
knowledge of a new method of charging metals with hydrogen 
at low temperatures. When a plate of zinc is placed in dilute 
sulphuric acid, hydrogen gas is liberated from the water by the 
oxidation of the metal, and it is evolved from the surface of the 
zinc, but no hydrogen is occluded. Mr. Graham remarks, “a. 
negative result was to be expected from the crystalline structure 
of zinc.” We are disposed to ask why crystalline structure should 
interfere with this power of retention ? If, however, a thin 
plate of palladium is immersed in the same diluted acid, and 
brought into metallic contact with the zinc, the hydrogen is 
transferred to its surface, and the gas is largely absorbed. The 
charge taken up in an hour by a palladium plate, rather thick, 
at 12° amounted to 173 times its volume. 
“ The absorption of hydrogen was still more obvious when 
the palladium plate was constituted the negative electrode, in 
acidulated water, to a Bunsen battery of six cells. The evolu- 
tion of oxygen gas at the positive electrode continuing copious, 
the effervescence at the negative electrode was entirely sus- 
pended for the first twenty seconds, in consequence of the 
hydiDgen being occluded by the palladium. The final absorp- 
tion amounted to 200 volumes.” 
The hydrogen enters the palladium and no doubt pervades 
the whole mass of the metal, but it exhibits no disposition to 
• Occlusion is a good old English word, signifying to ^ shut up,’ which 
had fallen out of uw, until Mr. Graham restored it as a scientific term. 
