v THE STUDY OF GASES 77 



held made no difference with respect to the red part of it ; 

 the part that was hottest being always of the deepest colour, 

 whether it was held upwards or downwards ; so that whether 

 the heated vapour ascended or descended, it did not retain 

 its colour in the smallest degree, after it had been opposite 

 to the heated part of the glass. 



" That this extraordinary redness was not occasioned by 

 the vapour being more rarefied in that particular place, 

 appeared by the whole tube assuming the same deep red 

 colour when the whole length of it was made equally hot : 

 for the vapour being closely confined, the density of it 

 within the tube must necessarily have continued the same 

 in all the variations of heat or cold. This redness, there- 

 fore, must be the proper effect of heat, on the phlogiston, 

 as I should imagine, of the vapour. Repeating this experi- 

 ment very often, with the same tube, and the same vapour, 

 it became alternately of a deeper or lighter colour, according 

 as it was kept hot or cold, without any sensible change, 

 except that which depended upon this single circumstance. 

 This is really a striking experiment, and especially when 

 the tube contains first so much vapour as to be nearly 

 transparent when it is cold ; so that the heat alone gives it 

 all the colour that it acquires. 



" In order to observe the utmost effect of heat on this 

 vapour, I placed the closed end of the tube near the fire, 

 and bringing it gradually nearer and nearer, observed that 

 the colour deepened uniformly with the increase of heat, 

 till, the glass actually melting, the compressed vapour burst 

 its way out" (Experiments on Air, 1777, III. 186-188). 



Priestley (1772) discovers " diminished nitrous air " or 

 " laughing gas." One of the agents used by Priestley 

 for " phlogisticating " or deoxidising common air was a 

 mixture of iron filings and brimstone, or sometimes iron 

 filings alone. The action of this agent on nitrous air gave 

 rise to a new gas possessing remarkable properties. Priestley 

 describes this discovery as follows : 



" The diminution of common air by a mixture of nitrous 

 air, is not so extraordinary as the diminution which nitrous 



