GENERAL OBSERVATIONS. To prosecute the chemical consideration of STEAMS farther than these general observations, which are applicable to all, would be almost to write a treatise of chemistry, and would be a repetition of many things which have been treated of in sufficient detail in other articles of this work. We shall therefore conclude this article with some other observations, which are also general, with respect to the different kinds of coercible vapours, but which have a particular relation to the following article.
Steam rises at different temperatures, according as the air is heavy or light. Steam or vapour is an elastic fluid, whose elasticity balances the pressure of the atmosphere; and it has been produced from a solid or liquid body raised to a sufficient temperature for giving it this elasticity; that is, for causing the fluid to boil. This temperature must vary with the pressure of the air. Accordingly it is found, that when the air is light (indicated by the barometer being low), the fluid will boil sooner. When the barometer stands at 30 inches, water boils at the temperature 212°. If it stands so low as 28 inches, water will boil at 208½°. In the plains of Quito, or at Gondar in Abyssinia, where the barometer stands at about 21 inches, water will boil at 195°. Highly rectified alcohol will boil at 160°, and vitriolic ether will boil at 88° or 89°. This is a temperature by no means uncommon in these places; nay, the air is frequently
warmer. Vitriolic ether, therefore, is a liquor which can hardly be known in those countries. It is hardly possible to preserve it in that form. If a phial have not its stopper firmly tied down, it will be blown out, and the liquor will boil and be dissipated in steam. On the top of Chimborazo, the human blood must be disposed to give out air-bubbles.
We said some time ago, that we had concluded, from some experiments made in the receiver of an air-pump, that fluids boil in vacuo at a temperature nearly 120 degrees lower than that necessary for their boiling in the open air. But we now see that this must have been ascends but a gross approximation; for in these experiments the fluids were boiling under the pressure of the vapour which they produced, and which could not be abstracted by working the pump. It appears from the experiments of Lord Charles Cavendish, mentioned in the article PNEUMATICS, that water of the temperature 72° was converted into elastic vapour, which balanced a pressure of ¼ths of an inch of mercury, and in this state it occupied the receiver, and did not allow the mercury in the gauge to sink to the level. As fast as this was abstracted by working the air-pump, more of it was produced from the surface of the water, so that the pressure continued the same, and the water did not boil. Had it been possible to produce a vacuum above this water, it would have boiled for a moment, and would even have continued to boil, if the receiver could have been kept very cold.
Upon reading these experiments, and some very curious ones of Mr Nairne, in the Phil. Trans. vol. lxvii. the writer of this article was induced to examine more particularly the relation between the temperature of the vapour and its elasticity, in the following manner: