Delicate Thermometers. 403 



at an}- other points. It may be objected, however, that the 

 capacity of the tube at these points, as given by the length 

 of a comparatively long (2 cm.) thread of mercury in that 

 position, affords but a very rough, and perhaps quite 

 erroneous, measure of the diameter of the tube at the parti- 

 cular point in question. As a crucial test, therefore, it was 

 determined to alter the bulb of one of the instruments. This 

 was done with Thermometer '39. The capacity of the bulb 

 was reduced to half its former value, so as to hold 18 instead 

 of 36 grams of mercury; and a determination of the coefficients 

 of expansion of the two bulbs underpressure (given in column 

 6 of Table II.) shows that what may be termed the apparent 

 expansion, or the effect of pressure on the height of the 

 mercury in the tube, was thereby reduced to nearly half its 

 former value, from *042 to *025 mm. per mm. of mercury- 

 pressure, so that the differences in the readings with falling 

 and rising columns should have been diminished in that 

 proportion, if it depended on the expansion of the bulb by 

 pressure; but on examination it was found that this difference 

 was even greater now than it had been previously — at 270 

 mm. it amounted to *59 instead of '27 mm., and at 457 mm. 

 it was '39 instead of *46 mm. (col. 7 of Table II.). Another 

 thermometer (No. '83) was then examined in a similar 

 manner. Originally it exhibited no difference whatever in 

 the felling and rising readings; but when the bulb was altered, 

 without increasing the coefficient of apparent expansion to 

 any considerable extent (from *011 to *015), a small though un- 

 mistakable difference in the readings was observed (No. '83 B 

 in the table). This second bulb was then removed, and a third 

 and smaller one substituted for it (No. '83 C), by which the 

 coefficient of apparent expansion was reduced by one half its 

 former value; but, instead of the differences being diminished, 

 they were actually increased, although no accurate measure 

 could be made of them, for the column of mercury in the 

 tube kept breaking off when the instrument was tapped. It 

 was evident, therefore, that the cause of these differences in 

 the readings did not lie in the bulbs of the instruments, but 

 in the steins, that each time the instrument was opened and 

 air admitted into the stem, the defect was increased, till the 

 tube eventually became entirely ruined. The moisture and 

 gases present in the air, no doubt, affect the glass and adhere 

 so strongly to it that the heating to which the stem is sub- 

 jected is quite incapable of removing it, and the interior of 

 the tube remains coated with an elastic covering which 

 destroys the working capabilities of the instrument. The 

 researches of Bottomley (Proc. Roy. Soc. xxxviii. p. 158) and 

 others on the absorption of air, and especially carbon dioxide, 



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