291. =Vaporization.=--The production of vapor by boiling differs in some respects from quiet evaporation. Here the liquid is raised in temperature to its boiling point, and the formation of vapor is not confined to the surface. In water the boiling point is 212°, but it varies more or less from this in other liquids. Thus the boiling point of alcohol is 173°, of ether 95°, oil of turpentine 568°, and mercury 652°. [Illustration: Fig. 209.] 292. =Influence of Pressure upon the Formation of Vapor.=--Pressure restrains the production of vapor, whether it be formed by evaporation or vaporization. We know by experiments with the air-pump that the less pressure of air there is upon the surface of a liquid the more rapidly will evaporation from it go on. I have already spoken of the influence of pressure upon the boiling of liquids in § 171. I will give here a few additional illustrations. Ether boils when it is heated to 95°, three degrees below the heat of the blood in our bodies. If we place some of it in a vessel under the receiver of an air-pump, by exhausting the air we can so take off the pressure that the ether will boil at the ordinary temperature of the air in a room. The restraint of pressure upon boiling is very strikingly shown in the _digester_, Fig. 209. This is a strong boiler, _a_, partly filled with water. A thermometer, _d_, is fastened into it so as to indicate the heat of the water. There is also a tube, _c_, extending to near the bottom of the boiler into a small quantity of mercury which is there. Let, now, the boiler be heated till the water boils, the air being left to escape by the stop-cock, _b_. If the stop-cock be shut, and we continue to apply the heat, we can raise the water to a very high temperature without having it boil at all, because of the pressure of the condensed steam upon its surface. An apparatus somewhat after this plan, called _Papin's digester_, has been used sometimes in cooking. The great heat to which water can thus be raised causes it to extract the nutritious matter from bones and cartilages, affording material for soup from what is commonly thrown away. To guard against the danger of explosion a safety-valve is provided, having a weight upon it which will keep it shut until a certain amount of pressure accumulates, and then it is forced open, letting out some of the steam. 293. =Steam.=--The cloud of steam, so called, which you so often see escaping from a locomotive is not really steam. Steam is transparent and invisible. You can see that it is so if you observe it issuing from the spout of a tea-kettle. It is only after it gets an inch or more from the spout that it becomes visible, and then it is really changed from steam into water by the condensing influence of the cold air. And the water in the cloud thus formed is probably in the same condition with the water in the clouds above, as described in § 288. [Illustration: Fig. 210.]