300 BURTON E. LIVINGSTON AND EDITH B. SHREVE 



standard surface, p is the corresponding maximum vapor-pres- 

 sure for the given field temperature, R is the time of response 

 observed in the laboratory (with maximum vapor pressure P), 

 and r is the calculated time of response for the field temperature. 

 According to this argument, the time required for color change 

 of a shp of paper over the standard surface at temperature t' 

 may be calculated from the corresponding time required by 

 the same paper over the same surface at temperature t. These 

 times may be called Rt and Rt {R and r in the formula just 

 given). We find (from tables) the maximum vapor-pressure 

 of water in the air for temperatures t and f (which may be called 

 Pt and Pt, which are P and p above) and then apply the formula, 



p Pt Rt 



Thus if a slip required 45 sec. to change over the standard 

 surface at 20°C. (vapor-pressure, 17.36 mm. of mercury), the 

 time required at 25°C. (vapor pressiu-e, 23.52 mm.) would be 



^ ^ 17-36 X 45 ^ 33 2 sec. 

 23.52 



The temperature t should be a function of the temperatures 

 of the evaporating surface of the rubber plate, and the glass 

 plate covering the paper slip, but if the apparatus has been ex- 

 posed for some time in the place where the test is made, it ap- 

 pears safe to consider that the temperature of the water just 

 behind the plate (as indicated by the thermometer in the labora- 

 tory type of apparatus) is a measure of the temperatm'e influ- 

 ence in question. In testing sUps in the laboratory, it is ad- 

 visable to keep the glass plate over the apparatus at all times 

 except when it is momentarily removed to place a slip. This 

 practically prevents a lowering of the temperature of the evapo- 

 rating surface through evaporation, and the whole apparatus 

 assumes approximately air-temperature. Such tests should be 

 made with air-temperature maintained as nearly constant as 

 possible. 



