354 PHYSICS. 



gram. The poteutial energy of liquid surfaces plays thus au imijor- 

 tant part in the great cycle-operations of nature, of which the author 

 gives some instances. {Nature, January, 1881, xxiii, 278.) 



WUllner and Grotrian have made observations which seem to prove 

 that the specific volume of vapors is independent of the size of the space 

 in which it is determined. The}^ confirm Herwig's result, that vapors 

 always undergo precipitation before reaching the so-called maximum 

 tension. Moreover, the tension at which condensation begins is found 

 to have a relation to the maximum tension which depends on the nature 

 of the liquid but is nearly independent of the temperature. Experi- 

 ments to find the degree to which vapors must be compressed to give the 

 maximum tension showed that there is no maximum tension in the sense 

 hitherto accepted; but that the tension of saturated vapors, even when 

 in contact with an excess of liquid, is perceptibly increased by compres- 

 sion. {Wied. Ann., 1880, II, xi, 545; Nature, February, 1881 xxiii, 307.) 



Wright has contrived a simple and convenient form of apparatus for 

 the distillation of mercury in vacuo, which is an improvement upon 

 those of Weinhold and Weber for the same purpose. A straight piece 

 of heavy glass tube, 5 or 6 millimeters interior diameter and rather more 

 than 70 centimeters long, is enlarged at one end to an oval bulb 85 mil- 

 limeters diameter and 120 millimeters long. To the upper end of this a 

 tube 15 millimeters interior diameter is joined, first rising 25 millimeters, 

 then inclined towards the bulb 130 millimeters, then sloping from the 

 bulb for 300 millimeters, and finally joined to a straight vertical tube 1 

 millimeter in diameter and 90 centimeters long. At the junction is a 

 lateral tube for connecting with the air-pump. The metal to be dis- 

 tilled is placed in a cistern beneath the 7G-'centimeter tube; the appara- 

 tus is exhausted, by means of a Sprengel pump, until the mercury from 

 the cistern reaches the bulb. Then the tube to the pumj) is sealed and 

 heat applied to tlie bulb very gradually. The vapor soon passes the 

 bend at the top and condenses beyond it, running down into the 90- 

 centimeter tube. At the bottom this tube is bent upward, a small bulb 

 is blown on it, and it is then bent horizontally. As the mercury falls in 

 the tube it maintains the exhaustion, the tube acting like a Sprengel 

 pump. The distillation is rapid, from 400 to 450 grams per hour being 

 easily obtained pure in this manner. (Am. J. ScL, December, 1881, III, 

 xxii, 479.) 



3. Conduction and radiation. 



Christiansen has employed the following simple method for some ex- 

 periments on heat conduction which he has made : Three round copper 

 plates were placed one above another and separated by small pieces of 

 glass. Into each plate a hole was bored radially, into which a ther- 

 mometer was inserted. The lowest plate rested on a brass vessel, 

 through which cold water is conducted. On the top plate rests a brass 

 vessel, through which warm water circulates. Through holes in the 



