C. A. Perkins — Vapor-tension of /Sulphuric Acid. 301 



Aet. XL. — Note on the vapor-tension of Sulphuric Acid, 

 with the description of an accurate Oathetometer Micro- 

 scope; by Chas. A. Peekins, Ph.D., Associate in Physics, 

 Bryn Mawr College. 



In the physical laboratory it is frequently necessary to meas- 

 ure small differences in the height of two objects, such as two 

 columns of liquid, etc., and especially to measure changes of 

 height ; and several forms of apparatus have been designed for 

 this work. The cathetometer is accurate for this purpose only 

 when provided with two telescopes and when the objects to be 

 measured are in the same vertical line, and then is not readily 

 used if the points are less than about 10 cm apart Desiring to 

 make some measurements of this nature, I made use of the 

 following apparatus : A microscope of rather long focus is 

 mounted upon a rigid vertical brass column and is carried by a 

 vertical micrometer screw, reading to f ^ mm . Immediately in 

 front of the object glass and covering one-half of it, is placed 

 a small total reflecting prism, so adjusted that when the micro- 

 scope is focussed upon one object, the prism throws into the 

 field the image of an object vertically above or below the 

 second, but on a level with the first. If the objects to be 

 measured are situated upon the same level, they will thus be 

 superimposed ; if not at the same level, they will come suc- 

 cessively to the center of the field, by moving the micrometer 

 through the necessary distance. With this apparatus, measure- 

 ments may be made in which the probable error of a single 

 setting is considerably less than -g^o- 1 *™ and in which the motion 

 is a simple vertical one as if the two objects were in the same 

 vertical line. The principal difficulty encountered is to bring 

 both objects into focus at the center of the field. For this it 

 is desirable to have one of the objects movable and to perform 

 the adjustment by moving this object, but it is quite possible 

 to make the adjustment when the objects are connected, as 

 the two legs of a U-tube. 



I have made use of this apparatus to measure the vapor 

 tension of sulphuric acid. This liquid has some properties 

 fitting it to be used in pressure gauges, either by itself or in 

 connection with mercury.* I desired to use it for this purpose 

 in a place where its vapor tension if appreciable would be 

 injurious. 



The only experiment, so far as I know, upon the vapor 

 tension, was made under the direction of Sir Wm. Thomson, 



* Callendar. On the practical measurement of Temperature, Phil. Trans., vol. 

 clxxviii, p. 161; and Bottornley, On a practical Air Thermometer with constant 

 volume, Phil. Mag., ser. V, vol. xxvi, p. 149. 



