iL'bl:h dc-jlycy Contagion. 399 



and the difficulty of diffufing it completely. But there is one 

 circumfhmee on which the Doctor appears to place fome reli- 

 ance, the truth of which we very much doubt, and that is, 

 that, during the difengagement of the vapour of the nitrous 

 acid, a confiderable quantity of pure air, or oxygene gas, is 

 at the fame time let Ioofe ; an opinion which is likewife fup- 

 ported by Mr. Keir. Now, from the generally received theory 

 refpecting the difference between the white and orange-colour- 

 ed vapour, (and it is the laft which is almoft uniformly gene- 

 rated in clofe veffels, or where pure air is excluded,) a quite 

 oppofite effect fliould take place, that is, the production of the 

 white vapour fliould be accompanied by a diminution of the 

 exygene gas in the room or apartment. In order to fatisfy my- 

 felf with regard to this fait, I made feveral experiments, the 

 remits of which were, that, when a cup containing a mixtuie 

 of heated nitre and concentrated lulphuric acid was placed 

 under a glafs jar inverted over water, the air in the jar, 

 after the nitrous vapour had been condenfed, was found to 

 have the proportion of its oxygene fomewhat diminiffied, the 

 difference being nearly as 93 to 90, or 31 to 30. This dif- 

 ference, which could not be perceptible in a large chamber, 

 having only the ufual quantity of nitrous vapour diflufed 

 through it, is but fmall, and I confefs much lefs than was 

 expected : it proves, however, that no oxygene gas is difen- 

 gaged. In thefe trials the jar, after fome time, was filled with 

 orange-coluLircd vapours; and this never failed to be the cafe, 

 where the heat of the materials, and ftrength of the fulphuric 

 acid, were fufficient to diferigagc any quantity of nitrous acid. 

 In the diftillationof this acid, il is true, that a little pure air 's 

 difengaged towards the end; but this never happens until the 

 bottom of the retort becomes red, and then the acid itfelf is 

 decompofed, which can never take place in Dr. Smyth's 

 procefs. 



By giving a place to the preceding remarks in your valua- 

 able Magazine, you will greatly oblige 



Yours, &c. kc. 

 XV ■"■ ■;■.;; V " : " W. Cjuicksiiank. 



