PHYSICS. 355 



efficient of depression of the body supposed anhydrous from 1*^ to 4°; 

 prolong the sensibly rectilinear part of this curve until it meets the 

 axis of ordinates ; multiply the ordinate of the point of intersection by 

 the molecular weight of the substance dissolved, supposed anhydrous ; 

 the product obtained will represent exactly the molecular depression 

 sought. (J. Phys., February, 1886, IT, v, 04.) 



Von Helmholtz has communicated to the Physical Society of Berlin 

 the method by which he determined the minimum diminution of vapor 

 pressure necessary to produce condensation of vapor, the heat being 

 constant. A glass cylinder was one third filled with the liquid to be 

 tested. Its upper portion, containing the mixture of air and vapor, was 

 connected on one side with a manometer, and on the other with a tap 

 through which the exhaustion could be effected. The formation of cloud 

 was detected by directing a beam of light through the axis of the cylinder, 

 and by looking along a line making a small angle with this axis, the eye 

 being screened from the direct light. At ordinary temperatures a de- 

 pression of 10'"™ of water was required to produce the cloud, while at 

 0° a depression of 12™ "^ was required. The statements of Coulier and 

 Aitken were confirmed, that the formation of cloud in saturated air was 

 induced solely by particles of dust. Saturated air, completely free of 

 dust, might suffer a depression of halfan atmosphere without formation 

 of any cloud without it. (IS'ature, April, 1886, xxxiii, 552.) 



Ramsay and Young have used an improved apparatus to determine 

 the vapor pressure of mercury. It consisted of a U tube inclosed in a 

 jacket containing the substance whose boiling point gave the tempera- 

 ture and connected with a manometer. The U tube was first filled 

 with mercury and boiled to expel air. Then on heating it and dimin- 

 ishing the pressure by means of a pump, vapor was evolved from the 

 mercury, which depressed the level on one side and raised it on the 

 other. From the difference of level in this tube and in a manometer 

 gauge connected with it, the vapor pressure was calculated. At 222.15° 

 the vapor pressure of mercury was found to be 34.4""" ; at 270.35° it was 

 124.35'""'; at 280.60°, 157.15'""'; 358.47°, 767.43"^-", and at 448°, 2904.5'"'". 

 (J. Chem. Soc, January, 1886, XLix, 37.) 



Vincent and Chajjpuis have determined in Cailletet's apparatus the 

 critical temperature and lyressure for two series of gaseous substances, 

 the members of each of which series differed in composition by CH^ and 

 showed a gradation of similar cliemical ])roperties. The first series con- 

 tained hydrogen chloride, methyl chloride, and ethyl chloride; the second 

 ammonia, methylamine,dimethylamine, and triraethylamine. For hydro- 

 gen chloride the critical temperature was 57.5° and pressure 96 atmos- 

 pheres ; for methyl chloride 111.5° and 73 atmospheres ; for ethyl chlo- 

 ride 54° and 182.5 atmospheres. For ammonia the values were for criti- 

 cal temperature 131° and pressure 113 atmospheres; for methylamine 

 155° and 72 atmospheres ; for dimethylamine 163° and 56 atmospheres, 

 and for trimethylamine 160.5° and 41 atmospheres. Hydrogen chloride 



