DETERMINATION OF CONDENSATION POINTS OF ENCLOSED GASES. 547 



spontaneously — a requirement Avhicli is met by the arrangement 

 shown, in which 



(A) is a lamp. 



(B) a glass globe holding about 1,500 c.c. of recently boiled 

 distilled water. 



(C) a thermometer graduated to one-tenth of a degree. 



(D) a perforated cork to hold the thermometer in any 

 required position. 



(E) the slide which should be in close contact with the 

 mercurial bulb of the thermometer. 



(F) a flat piece of glass which must be " oiled on " to the 

 globe at the point of contact. 



(G) a low-power objective which forms a telescopic image 

 of the slide at the point H. 



(M) a microscope with which to observe the image H. 



To use this apparatus the flask is filled with water at a 

 temperature of 85° to 86° F. 



The microscope and lamp are placed at opposite sides of the 

 globe and as nearly as possible in a line passing through its 

 centre. The flat piece of glass is then oiled on and secured in 

 its position at right angles to this line. 



Under these conditions the definition of an object in the 

 centre of the globe will be sufliciently good to enable the 

 cavities to be satisfactorily examined. The thermometer is 

 then lowered into its place as close to the- object as is 

 possible. 



The object of employing such a large bulk of water is to 

 prevent the lowering of the temperature proceeding too rapidly, 

 thereby enabling the thermometer and slide to approximate to 

 each other's temperature, and to compensate in some measure 

 for the different rates at which different bodies radiate heat 

 into their cooler surroundings. 



In practice a fall of one-tenth of a degree F. per minute should 

 not be exceeded, and as this very gradual and uniform change of 

 temperature is so necessary, it follows that the reverse operation 

 of raising the temperature by means of a lamp placed beneath 

 the globe gives fallacious results ; for however carefully done, 

 the thermometer cannot quickly respond to the temperature 

 of the mixed currents of hot and cold water which are inevitably 

 produced, but by following the method here recommended 



