Thermodynamic Properties of Air. 295 



80 cm. in length, on which five bulbs of nearly equal capacity 

 have been blown ; the connecting-pieces are provided with 

 divisions in millimetres. The eudiometers are surrounded by 

 wide cylindrical jackets of clear glass filled with water. Two 

 thermometers, 6 X and 2 , immersed in the water give the 

 temperature of the gas ; air-streams blown in through p' 

 and p" serve to equalize the temperature of the water. 



The eudiometers are connected by means of thick-walled 

 indiarubber tubing with open mercury-manometers M x and 

 M 2 , fixed to two sliding pieces on a vertical wooden support 

 (rot shown in the figure). 



The calibration of the eudiometers was effected after they 

 were put in their place and surrounded with water. To do 

 this by mercury-weighing, the glass stop-cocks near the lower 

 ends of the eudiometers were provided. Positions of the 

 mercury meniscus were read by means of a cathetometer, placed 

 in a position where it was to remain during the rest of the 

 experiments. In this manner both eudiometers were cali- 

 brated twice. From the results of calibration I prepared a 

 table giving capacities in cubic millimetres (at the standard 

 temperature of + 17° C.) corresponding to every scale- 

 division. Each of the five glass bulbs had a capacity of nearly 

 40 c.cm., the whole eudiometer about 210 c.cm. One milli- 

 metre on the glass necks corresponded to 60 c.mm. capacity. 

 Therefore the smallest volume which could be appreciated was 

 about 3 c.mm. 



After calibration of the eudiometers, I made another set of 

 measurements in order to determine the capillary depres- 

 sions &c. These corrections (of small importance) have been 

 determined for every scale-division in an obvious manner. 



§ 5. Preparation of Gas. — To complete the description of 

 the apparatus, a few words remain to be said about the reser- 

 voir of compressed air and the mode of compressing and 

 purifying the gas. One of the well-known carbonic-acid 

 bottles of 11 litres capacity, resisting 250 atmospheres, is 

 used as air-store. The usual screw-valve is replaced by 

 a simple stopper of brass with a copper tube soldered in it, 

 connecting the reservoir Z with two drying-tubes S', S". 

 These are made of steel, and are filled with finely grained 

 chloride of calcium and potassium hydroxide, stoppered by 

 thick layers of cotton-wool. A considerable quantity of 

 these substances is also contained in the reservoir Z in a 

 vertical tube of wire-gauze. The air submitted to experi- 

 ment is compelled to pass the drying - tubes twice — during 

 the charging of the reservoir, and again on its way to the 

 apparatus. Besides that, it remains a long time in contact 

 with the drying-substance in the reservoir itself. 



To condense the gas, a Natterer condensing-pump has been 



