98 Mr. Shelford Bidivell [May 5, 



spot of light to the left indicates cold, to the right, heat. When the 

 stop-cock is opened so that a portion of the air escapes from the flask 

 into the air-pump receiver, you see at once a violent movement of 

 the spot of light to the left, showing that the expansion of the air is 

 accompanied by a fall of temperature. If more air from the room 

 is allowed to enter the flask, the spot moves in the opposite direction. 



The large glass globe, upon which the beam from the electric 

 lantern is now directed, contains ordinary air, kept in a state of 

 saturation, or nearly so, by the presence of a little water. You will 

 observe that although heavily laden with water vapour the air is 

 perfectly transparent. If, now, we turn a tap and so connect the 

 globe with the exhausted receiver, the air expands and becomes 

 colder ; the space inside the globe is no longer able to hold the same 

 quantity of water as before in the form of vapour, and the excess is 

 precipitated as very finely divided liquid water — water dust it may be 

 called — which fills the globe and is perfectly visible as a cloud or 

 mist. In a few minutes the cloud disappears, partly, no doubt, 

 because some of the particles of water have fallen to the bottom of 

 the vessel, but chiefly because the air becomes in time warmed up 

 to its original temperature (that of the room), and the suspended 

 water is converted back again into invisible vapour. 



Now let us repeat the experiment, and before the cloud has time 

 to disperse let us admit some fresh air from outside ; the cloud, as 

 you see, vanishes in an instant. The compression of the air raises 

 the temperature above the dew point, and the small floating particles 

 of water are transformed into invisible vapour. 



I once more rarefy the air, and admit a fresh supply while holding 

 the flame of a spirit-lamp near the orifice of the inlet pipe, so that 

 some of the burnt air is carried into the interior of the globe. When 

 the air is again expanded a cloud is formed which is, as you observe, 

 far more dense than the others were. It appears on examination 

 that the increased density of this cloud is not due to the condensa- 

 tion of a greater quantity of water. Little, if any, more water is 

 precipitated than before. But the water particles are now much more 

 numerous, their increased number being compensated for by dimin- 

 ished size. Within certain limits, the greater the number of particles 

 into which a given quantity of water is condensed, the greater will 

 be the apparent thickness of the mist produced. A few large drops 

 will not impede and scatter light to the same extent as a great 

 number of small ones, though the actual quantity of condensed water 

 may be the same in each case. 



Then comes the question, why should the burnt air from the 

 flame so greatly increase the number of the condensed drops ? An 

 answer, though perhaps not quite a complete one, is furnished by 

 some remarkable experiments made by M. Coulier, a French pro- 

 fessor, nearly twenty years ago. He believed his experiments 

 pointed to the conclusion that water vapour would not condense at 

 all, even at temperatures far below the clew point, unless there were 



