108 Some Particulars respecting the 
voluntarily ; both hands came out victoriously from the proof. 
And now, if anything surprise me, it is that such experiments 
are not quite common. 
I will surely be asked what precautions must be taken to 
preserve one’s self from the disorganising action of the in- 
candescent matter. I answer, None. Have no fear; make 
the experiment with confidence, pass the hand rapidly, but 
not too rapidly, through the stream of fully melted matter. 
Otherwise, if the trial be made timidly, or with too great 
haste, we may overcome the repulsive force which exists in 
the incandescent bodies, and thus bring them in contact with 
the skin, which will then unquestionably be reduced to a 
state that may be easily imagined. 
To perceive the danger that must arise from passing the 
hand too rapidly through the melted metal, we have only to 
recollect, that the resistance is in proportion to the square of 
the quickness, and in a compact fluid, such as melted iron, 
this resistance certainly increases in a greater ratio. 
The experiment succeeds more especially when the skin 
is moist ; and the involuntary alarm we feel beside these 
masses of fire almost always brings the body into that state 
of moisture so necessary to success. But by taking a few 
precautions, we truly become invulnerable. 
The following method succeeded best with me: I rubbed 
my hands with soap, so as to give them a polished surface, 
then at the moment of making the experiment, I plunged the 
hand into a cold solution of sal ammoniac, saturated with 
sulphuric acid, or simply into water containing sal ammoniac, 
and when that was not at hand into cold water. 
Regnault, who has investigated this subject says: “Those 
who make a trade of handling fire, and holding it in their 
mouth, sometimes use an equal mixture of spirit of sulphur, 
sal ammoniac, essence of rosemary, and onion juice,” all 
volatile substances, it will be observed, which, while evapo- 
rating, render a certain portion of heat latent.* 
A common experiment in glass-houses, with which I was 
made acquainted by M. Dumas, may be mentioned here. It 
* Le P. Regnault, Entretiens sur la Physique, t. xi., p. 101, edit. of 1737. 
