95 
use some acid which had only just been diluted and was 
warm ; then the gas came off twenty or thirty times as fast 
as it had previously done. I then put a lamp under the 
bath and measured the rate at which the gas came off, and 
I found that when the acid was on the point of boiling as 
much hydrogen was given off in 5 seconds as had previously 
come off in 10 minutes, and the rate was maintained in both 
cases for several hours. 
After havino’ been in acid for some time the tube was 
O 
taken out, well washed with cold water and soap so as to 
remove all trace of the acid ; it was then plunged into a 
bath of hot water, upon which gas came off so rapidly from 
both the outside and inside of the tube as to give the 
appearance of the action of strong acid. This action lasted 
for some time, but gradually diminished. It could be 
stopped at any time by the substitution of cold water in 
place of the hot, and it was i*enewed again after several 
hours by again putting the tube in hot water. The volume 
of hydrogen which was thus given off by the tube after it 
had been taken out of hot acid was about equal to the vo- 
lume of the iron. 
At the time I made These experiments I was not aware 
that there had been any previous experiments on the sub- 
ject; but I subsequently found, on referring to Watt's 
Dictionary of Chemistry, that Cailletet had in 1868 disco- 
vered that hydrogen would pass into an iron vessel immersed 
iu sulphuric acid. See Comp. rend. Ixvi, 847. 
The facts thus established appear to afford a complete 
explanation of the effects observed by Mr. Johnson. 
In the first place, with regard to the temporary character 
of the effect, it appears that hydrogen leaves the iron slowly 
even at ordinary temperatures — so much so that after two 
or three days’ exposure I found no hydrogen given off when 
the tube was immersed in hot water. Witli regard to the 
effect of warming the wire — at the temperature of boiling 
