5. _Its Dryness or Humidity._--Evaporation from the waters of the globe, from seas lakes, rivers, etc., is perpetually taking place, the ratio of evaporation being determined by the temperature of the atmosphere; that is, it increases as the temperature increases. Hence the actual quantity of water in a state of invisible vapour in the atmosphere will be the greatest when the temperature of the latter is the highest. Speaking according to our feelings, we then call the air dry, although, in fact, the quantity of water in solution in the atmosphere may balance the demands of the temperature.[5] But this invisible vapour is liable from a thousand causes to condensation, dependent upon one law, namely, that when a portion of air, saturated with invisible vapour, is cooled below the point of saturation, a portion of the vapour becomes condensed, according to the degree of cold, so as to leave a due balance of vapour in the air, according to its altered temperature. Who has not seen the window-panes of a heated room in cold weather bedewed with trickling moisture? The vapour of the room is in abundance, although invisible; but certain strata of the air thus charged come in contact with the cold panes, and lose a portion of their caloric, and with their caloric a portion of their vapour, (to which, perhaps, the respiration of a large party has contributed,) and this becomes condensed in the form of a visible fluid. If we breathe against a cold pane of glass, we soon render it dim by moisture. On a hot summer's day, we do not perceive the vapour of our breath; but on a cold, clear, frosty day, a visible vapour or little mist is exhaled. [5] The quantity of water in solution in the atmosphere can never be greater than the quantity proper to the temperature; but it may be less.