PHYSICS. 243 



of ODetentli of an inch of the mercurial column is more than an inch on 

 the glycerin scale. Moreover, the high boiling point of glycerin, 440° 

 F., its very low vapor tension, and its low freezing ])oiut are all in its 

 favor. The cistern in the Kew instrument is a cylindrical vessel of tinned 

 copper, 10 inches diameter and 5 deej), with a screw cover, the air having 

 access through a small hole in the cap of this, and passing through a 

 layer of cotton wool to keep out the dust. The tube is of composition- 

 metal gas-pipe ^ inch diameter, which is soldered to a similar piece of 

 pijie which enters the cistern through its bottom and is closed by a 

 screw plug. To the top of the main tube, a glass tube 4 feet long and 

 1 inch inside diameter is cemented. It terminates in a cup and is closed 

 with an India-rubber stopper. The lluctuations are read oif by means 

 of two verniers, one of which gives inches and tenths and the other the 

 equivalent value of the mercurial column. Daily observations are being 

 made upon it with a view to determine its accuracy. — {Xaturc, xxi, 377, 

 1880.) 



Terquem has proposed a modified form of air-pump plate, which dis- 

 penses with the costly grinding necessary in the present form. The plate 

 is of brass, circular in form and about a millimeter in thickness, having 

 a circular cavity in it two centimeters deep and one wide, placed a few 

 centimeters from the edge, the mean diameter of this depression being 

 that of the bell to be used with it. When the plate is to be used it is 

 heated and the cavity tilled with a cement melting at about 60^ C. That 

 used by the author is furnished by F. Carre with his ice-pump, and con- 

 sists of a mixture of resin, wax, and tallow. When melted the bell is 

 placed in i^osition and the heat continued until the glass has also be- 

 come hot. The whole is then allowed to cool. The cheapness of these 

 plates allows the use of several of them for the various air-pump experi- 

 ments. — {J. Fhys., ix, 346, October, 1880.) Rood has descnbed an im- 

 provement in the form of the Sprengel ijump, by which vacua as high 

 as 900-wooo or T^ 0^00 are readily obtained. Fii^st, the mercury 

 passes through an exhausted bulb to free it from air and inoisture, then 

 through a nearly horizontal tube to the fall-tube. Secondly this fiill 

 tube is bent four times at a little more than a right angle, forming a 

 lateral loop or valve, preventing the air that has passed out of the fall- 

 tube from returning into it. The pump i s free from stopcocks and grease, 

 and its action is very rapid, two or three hours sufficing to reduce the 

 vacuum from 50 M to 0.01 M, the capacity being 100<"°. Even in the 

 absence of all drying materials a vacuum as high as 0.033 M has been 

 obtained. — {Am. J. ScL, III, xx, 57, July, 1880.) 



Bertin, in an interesting article on the modern methods for introducing 

 high vacua, describes the McLeod gauge, that necessary adjunct to the 

 Sprengel pump. It depends on ^e principle that if the volume of a 

 rarefied gas be reduced n times, the pressure becomes measurable, and 

 the original pressure is equal to the measured pressure divided by n. It 

 consists of a bulb of glass surmounted by a thick tube 10 to lo"-^ long 



