I have tried fixed air as an antiseptic taken in by respiration, but with no great success. In one case it seemed to be of service, in two it seemed indifferent, and in one was injurious, by exciting a cough. NUMBER V. _Extract of a Letter from Mr. WILLIAM BEWLEY, of GREAT MASSINGHAM, NORFOLK._ March 23, 1774. Dear Sir, When I first received your paper, I happened to have a process going on for the preparation of _nitrous ether_, without distillation.[25] I had heretofore always taken for granted that the elastic fluid generated in that preparation was _fixed_ air: but on examination I found this combination of the nitrous acid with inflammable spirits, produced an elastic fluid that had the same general properties with the air that you unwillingly, though very properly, in my opinion, term _nitrous_; as I believe it is not to be procured without employing the _nitrous_ acid, either in a simple state, or compounded, as in _aqua regia_. I shall suggest, however, by and by some doubts with respect to it's title to the appellation of _air_. Water impregnated with your nitrous air _certainly_, as you suspected from it's taste, contains the nitrous acid. On saturating a quantity of this water with a fixed alcali, and then evaporating, &c. I have procured two chrystals of nitre. But the principal observations that have occurred to me on the subject of nitrous air are the following. My experiments have been few and made by snatches, under every disadvantage as to apparatus, &c. and with frequent interruptions; and yet I think they are to be depended upon. My first remark is, that nitrous air does not give water a sensibly acid impregnation, unless it comes into contact, or is mixed with a portion of common or atmospherical air: and my second, that nitrous air principally consists of the nitrous acid itself, reduced to the state of a _permanent_ vapour not condensable by cold, like other vapours, but which requires the presence and admixture of common air to restore it to its primitive state of a liquid. I am beholden for this idea, you will perceive, to your own very curious discovery of the true nature of Mr. Cavendish's _marine_ vapour.