452 L. Waldo— Papers on Thermometry, 
e now come to my criticism of the Geissler thermometer for 
not having a reservoir at the top. Dr. Thiesen has in some way 
misunderstood my principal reason for its presence. My reason 
was not that “es vermindert die Schidlichkeit der im Quecksilber 
et, Spee von Luft” but that only by its use can 
the mercury in be entirely free from air, Take a ther- 
no bt and La it with the bulb on top. If the thermometer 
s large, in nine cases out of ten the mercury will separate and 
fall down: allow it to remain and observe the bubble-like spel 
um in the bulb. Turn the bulb in various directions so as to, 
it were, wash the whole interior of the bulb, and then bring t the 
thermometer into a vertical position, keepin the bubble in 
sight. As the mercury flows back, the bubble diminishes and 
finally, in a good thermometer, almost disappears: but in most 
thermometers a good sized bubble of air, in some cases as large 
as the wire of a pin, remains. It is the most important function 
of a reservoir at the top to permit such manipulations as to drive 
all such air into the top reservoir and to make the mercury and 
the glass assume such perfect contact that the bulb can be turned 
uppermost without the mercury separating, even in thermometers 
of large size and with good generous bulbs. In many Geissler 
thermometers such a test might succeed, not on account of the 
eet to prevent the fall, Now TI think that a thermometer in 
ich there is this layer of air around the merebiry in the bu 
inst be uncertain in its action; hence my opinion is unaltered 
that all thermometers in which we e cannot aac a this layer or at 
least make certain of its absence should be reje 
Furthermore, with respect to ci hye the reservoir is not 
essential to the ese eat of thermometers "whose ee is on and 
mercury can be stored up in the reservoir so as to allow the col- 
umn to move over the whole scale. And it is within this limit 
_ soap ip are of the greatest value in the physical labo- 
rat 
and de sewed water to nter,, the ‘small Silieg tube 
the icake all these were so obvious shee I ahaa my remarks 
to the more obscure errors : 
Furthermore, I believe there is some error in most Geissler 
thermometers from the small size of the bulb and the capillary 
tube, and this I have mentioned on p. 124 of the paper referred 
to. Pfaundler and Platter, in a paper on the specific heat of 
water, in Poggendorff’s Annalen for 1870, found an immense 
variation within small limits. Ina subsequent paper* the authors 
* Poggendorff’s Annalen, exli, p. 537. 
