324. =Clouds and Latent Heat.=--The water of which clouds are composed is heavier than air. Why, then, does it remain suspended? Why is it necessary that it should be collected into drops in order to have it fall? This question can be answered by looking at the manner in which clouds are formed. A cloud, I have stated in § 288, is made up of minute vesicles or bubbles containing air. Now the air in these bubbles is lighter than the air that is around the cloud, because it is warmer. But how does it get its heat? In order to understand this observe what the bubble is made from. It is made from the water which was in the air in a state of vapor, or in its aeriform state, for this is the state of water that is evaporated and dissolved in the atmosphere. But when it forms the vesicle it goes out of this state and becomes a liquid, for the wall of the vesicle is a liquid wall, just as the wall of a soap-bubble is. Now in passing out of the aeriform into the liquid state some latent heat must be made sensible. What becomes of this sensible heat? It just heats the air in the vesicle, and so makes it like a heated air-balloon. So all clouds are collections of innumerable heated air-balloons, and the reason that some clouds are higher up than others perhaps is that their balloons have warmer and therefore lighter air in them. 325. =Freezing Mixtures.=--The intense cold produced by these mixtures is the result of the change of free or sensible heat into latent. For example, when salt and snow are mingled together a melting of the two is quickly produced. In this sudden change of a solid into a fluid a great quantity of heat must be rendered latent, and therefore there will be a great loss of sensible heat by whatever the freezing mixture comes in contact with. The process here, you see, is the opposite of solidification in relation to latent heat. A portion of the snow, after melting with the salt, becomes solid ice. Why is this? It is because it gives up its sensible or free heat to portions of the snow that are in the process of melting and are therefore making heat latent.