402 Mr. W. H. Pendlebmy and Miss M. Seward. 



known strength, together with 10 c.c. of potassium iodide solution 

 containing O'l gram of the salt and 10 c.c. of clear starch solution, 

 mixture being rapidly made after each addition by the passage 

 of large bubbles of carbon dioxide from the inverted funnel. 

 These bubbles of gas, with a diameter equal to half that of the 

 cylinder, served to stir the liquid and also to exclude the air. A 

 few drops of a dilute solution of thiosulphate were added to keep 

 down the blue colour till all was prepared for observation. There 

 was a line scratched on the funnel stem, and this mark and one of the 

 graduations of the thermometer were made to coincide with the plane 

 of the line round the cylinder. The temperature of the liquid was 

 brought up to the required point, then the cylinder was placed on a 

 levelling stand, and water was added till the lower surface of the 

 meniscus just coincided with the plane of the marked line. Mean- 

 while a number of small measures of a concentrated solution of sodium 

 thiosulphate had been prepared. These measures must be equal or 

 have a known ratio to each other ; they must also be of small volume, 

 in order that their addition may not materially affect the dilution of 

 the liquid. These measures were obtained in the following way : A 

 series of tubes about 8 inches long, having a lateral orifice about 

 1J inches from the end, such as would be made for the purpose of 

 joining on another tube at right angles, were mounted on a carriage, 

 each tube having a separate rest, and all the orifices being in one line. 

 By the tarn of a screw connected with a rack and pinion these tubes 

 could be brought exactly under a siphon delivering drops of thio- 

 sulphate. The siphon and its reservoirs stood on a bracket attached 

 to a pillar of solid masonry to prevent vibration. The whole was en- 

 closed in a glass case like that of a balance, the front of which was 

 shut down during the time of collection of the drops. The time 

 of formation of a drop was generally about half a minute. The width 

 of the reservoir containing the thiosulphate is so great in com- 

 parison with the quantity of solution taken for any one set of experi- 

 ments that the available length of the siphon and the rate of flow, 

 upon whose constancy that of the drops depends, varies in no appre- 

 ciable degree. At the end of each experiment the value of the 

 drops employed was determined by means of a standard iodine 

 solution. 



When the observations were to be made the cylinder was placed 

 on a sheet of white paper in a good light, opposite a clock beating 

 seconds. The paper lay on an iron plate, which could be heated at 

 once, and by a lamp if necessary, and thus the cylinder could be 

 kept at any desired temperature by moving it nearer to or further 

 from the heated end of the plate. When once the most convenient 

 spot has been selected, a mere touch with the hand was all that was 

 required to maintain the temperature constant. 



