[14] I have not repeated this experiment with that variation of circumstances which an attention to Mr. Bewley's observation will suggest. SECTION IV. _Of MARINE ACID AIR._ In my former experiments on this species of air I procured it from spirit of salt, but I have since hit upon a much less expensive method of getting it, by having recourse to the process by which the spirit of salt is itself originally made. For this purpose I fill a small phial with common salt, pour upon it a small quantity of concentrated oil of vitriol, and receive the fumes emitted by it in a vessel previously filled with quicksilver, and standing in a bason of quicksilver, in which it appears in the form of a perfectly _transparent air_, being precisely the same thing with that which I had before expelled from the spirit of salt. This method of procuring acid air is the more convenient, as a phial, once prepared in this manner, will suffice, for common experiments, many weeks; especially if a little more oil of vitriol be occasionally put to it. It only requires a little more heat at the last than at the first. Indeed, at the first, the heat of a person's hand will often be sufficient to make it throw out the vapour. In warm weather it will even keep smoking many days without the application of any other heat. On this account, it should be placed where there are no instruments, or any thing of metal, that can be corroded by this acid vapour. It is from dear-bought experience that I give this advice. It may easily be perceived when this phial is throwing out this acid vapour, as it always appears, in the open air, in the form of a light cloud; owing, I suppose, to the acid attracting to itself, and uniting with, the moisture that is in the common atmosphere. By this process I even made a stronger spirit of salt than can be procured in any other way. For having a little water in the vessel which contains the quicksilver, it imbibes the acid vapour, and at length becomes truly saturated with it. Having, in this manner, impregnated pure water with acid air, I could afterwards expel the same air from it, as from common spirit of salt.