234 Dr. 0. Masson. On the Wetting of [Apr. 25, 



temperature, then oscillates for some time about that point (following 

 the oscillations of the gas regulator) and finally rises steadily to the 

 full temperature. The effect is seen best when absorbent wool is used, 

 as it carries a much larger store of water. 



The immersion bath varied according to circumstances. In Series II, 

 for water immersion, it was simply a glass jar holding about 3 litres of 

 distilled water. Its temperature was that of the laboratory and was 

 thus not the same throughout the series, but it varied only between 

 16 and 18, and during any one experiment it did not alter more than 

 a few tenths of a degree. This last variation was recorded and after- 

 wards approximately corrected for, in plotting the temperature curve 

 of the covered thermometer. The results are, of course, though fairly 

 good, less reliable than those of later series ; for in all of these the 

 immersion bath was placed in a thermostat which kept constant to 

 within 0'03. In the later water immersion tests the bath was a wide- 

 mouthed thin glass flask containing distilled water, and it was clamped 

 in the thermostat so that only its open mouth was above the surface of 

 the water in the latter. For immersion in air saturated with water 

 vapour, this flask was replaced by another arrangement which under- 

 went some modification as the work proceeded. In Series III a large 

 wide-mouthed glass bottle was used, with a layer of water at the 

 bottom and with an air current passing through it. This air was 

 previously saturated at the right temperature by passing through a 

 lead worm and a wash-bottle, both submerged in the thermostat, and 

 it was then delivered into the bottle through a tubulated neck near the 

 bottom. In Series IV the air current was sent through additional 

 saturators, and a cover was provided for the bottle so as to close it 

 when in use, except for the opening which admitted the thermometer. 

 In Series V, as some doubts were entertained as to perfect saturation 

 in previous tests, the bottle was replaced by a large cylindrical pot of 

 porous earthenware, with a hole near the bottom through which the air 

 current could be conveyed. The pot was very completely surrounded 

 by the water of the thermostat and was provided with perforated card 

 covers. The infiltration of water through the walls kept the whole 

 inside surface wet and provided a large and constant area of evaporation. 

 At the same time the infiltration was not rapid enough to give any 

 trouble, even in protracted experiments, if the pot was emptied by a 

 syphon at the start. With this arrangement it was found that it made 

 no appreciable difference whether the air current was used or not, and 

 it was not used in the last six experiments of the series. 



Hooks, which occupied a fixed position with respect to the telescope 

 so that its distance and the value of its scale did not vary, served for 

 the vertical suspension of the thermometers during immersion. The 

 small water motor which worked the stirrer of the thermostat was 

 connected with the same support as these hooks, and at each rotation 



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