120 Henry Riddell on 



vapour, one succession of three by a method which he beh'eved 

 would give values too high in amount, and five by another method 

 which he was equally certain would give a figure too low. If we 

 take the mean of those due to the first method and also of the 

 five in the second series and use half the sum of these as the most 

 probable value due to his experiments we find the resulting figure 

 to be 537°, which means that the amount of heat required to 

 convert say one pound of water at 100° into steam also at 100° 

 is the same as would suffice to raise 5-37 pounds of water through 

 the whole range between 0° and lOO^C. But these figures are 

 calculated on the understanding that the mean specific heat was 

 constant. When they are recalculated in the light of modern 

 knowledge, and of the most accurate experiments now on record, 

 the figure of 537° becomes 538-3°, and we note also that the most 

 modern value chosen for this quantity is 539-3°, arising out of 

 experiments conducted with all the refinements of electric methods 

 by which temperature can be measured to about one-thousandth 

 of a degree. When Dr. Andrews made his experiments it was 

 necessary to read the mercury thermometer to less than one 

 one-hundredth of a degree to measure the variation between 

 538° and 539-3°. 



I should next mention the series of experiments on Ozone, 

 and here a short introduction is necessary. It had been noticed 

 that a body having peculiar odour was formed under the following 

 different circumstances, and that in each case its properties 

 appeared to be similar : — 



1. Passing electric si)arks through air. 



2. When water is decomposed by the electric current this 

 substance appears at the positive pole along with oxygen. 



3. By the slow oxidation of Phosphorous in air at ordinary 

 temperatures. 



For a long time the composition was uncertain, and it required 

 experiment to determine if the same substance was formed in 

 each of these cases. 



It was believed by many chemists that the substance obtained 



