TOUCHWOOD TINDER-BOX. 105 



be produced several times in succession, merely by pushing the 

 piston with the hand. This vapour is so thin and diaphanous, that 

 it is not perceptible in a strong light. It requires a sort of tw ilight 

 to see it well. 



li But whence arises this vapour, and what is i(s nature ? As. 

 suredly it is not furnished by the materials of the instrument ; it 

 can only proceed, therefore, from what it contains, from the atmo- 

 spheric air. Now, according to the present state of our knowledge, 

 the air contains only nitrogen, oxygen, and a very small portiou 

 of carbonic acid; all gassiform substances, which are kept in this 

 state by the great quantity of caloric that penetrates them, and arc 

 consequently heavier than it. But in compressing the air con. 

 tained in the tube, what is the substance that must first give way ? Is 

 it not that which is lightest, the caloric, that general solvent, that 

 principle of fluidity and volatilization, which gives wings even to 

 metals to raise themselves in the air ? Is then the vapour in ques- 

 tion caloric, rendered visible by the approximation of its particles, 

 which are compressed by the surrounding air, as air becomes visible 

 in passing through liquids ? This idea, which I am far from pre- 

 senting as a thing proved, acquires more probability from the fol. 

 lowing experiments. 



" I substituted hydrogen for common air, and the vapour 

 showed itself as before j but the spunk did not take fire. With 

 carbonic acid gass, and with nitrogen, the effects were the same. 

 The latter, which contained a little nitrous gas, gave a somewhat 

 denser vapour. Oxygen, lightly compressed, yielded a vapour 

 more rare and transient than that from common air. It had 

 scarcely fallen on the piston when it rebounded and disappeared. 

 "When I compressed oxygen with a proper force for producing in. 

 flammation, the spunk, which commonly takes fire only at the 

 anterior part, was almost entirely burned : yet for this experiment 

 I used a copper instrument, the piston of which lost air so much, 

 that it would no longer kindle spunk (with common air). 



u Perhaps it will be said, that the vapour came from the greasy 

 matter on the piston, which adheres to the sides of the tube ; and 

 that it is expanded by the heat produced by the friction. To this 

 I answer, in this case, 1st. The vapour should not shew itself be* 

 fore the greasy matter is deposited on the sides of the tube ; yet it 

 appears at the first stroke of the piston, before the tube becomes 

 greasy. 2dly. It should show itself below the piston, in the part 



