268 



would produce a deformation, or at least a compression of the glas 

 that would materially aflPect the indications. For a thermometer 

 of exti'eme sensibility, mercury does not appear to be a conve- 

 nient liquid ; since, if a very fine tube be employed, there is 

 some uncertainty in the indications on account of the irregularity of 

 capillary action, due probably to superficial impurities, and observ- 

 able even when the best mercury that can be prepared is made use 

 of ; and again, if a very large bulb be employed, the weight of the 

 mercury causes a deformation which will produce a very marked 

 diff'erence in the position of the head of the column in the tube ac- 

 cording to the manner in which the glass is supported, and may 

 therefore affect with uncertainty the indications of the instrument. 

 The former objection does not apply to the use of any fluid which 

 perfectly wets the glass ; and the last-mentioned source of uncer- 

 tainty will be much less for any lighter liquid than mercury, of equal 

 or greater expansibility by heat. Now the coefficient of expansion 

 of sulphuric ether, at 0° C, being, according to M. I. Pierre,* 

 •00151, is eight or nine times that of mercury (which is -000179, 

 according to Regnault) ; and its density is about the twentieth part 

 of the density of mercury. Hence a thermometer of much higher 

 sensibility may be constructed with ether than with mercury, without 

 experiencing inconvenience from the circumstances which have been 

 alluded to. An ether thermometer was accordingly constructed by 

 Mr Robert Mansell of Glasgow, for the experiment which I pro- 

 posed to make. The bulb of this instrument is nearly cylindrical, 

 and is about 3|^ inches long and f th of an inch in diameter. The 

 tube has a cylindrical bore about 6^ inches long : about 5 J inches 

 of the tube are divided into 220 equal parts. The thermometer is 

 entirely enclosed, and hermetically sealed in a glass tube, which is 

 Just large enough to admit it freely. On comparing the indications 

 of this instrument with those of a thermometer of Crichton's with 

 an ivory scale, which has divisions, corresponding to degrees Fahren- 

 heit, of about o^g th of an inch each ; I found that the range of the 

 ether thermometer is about 3° Fahrenheit ; and that there are 

 about 212 divisions on the tube corresponding to the interval of 

 pressure from 31° to 34°, as nearly as I could discover from such 

 an unsatisfactory standard of reference. This gives y^x ^^ ^ degree 



* See Dixon on Heat, p. 72. 



