124. Prof. Thomson on the Effect of Pressure in 



To test the phaenomenon by experiment without applying 

 excessively great pressure, a very sensitive thermometer would 

 be required, since for ten atmospheres the effect expected is 

 little more than the tenth part of a Fahrenheit degree; and 

 the thermometer employed, if founded on the expansion of a 

 liquid in a glass bulb and tube, must be protected from the 

 pressure of the liquid, which, if acting on it, would produce a 

 deformation, or a least a compression of the glass that would 

 materially affect the indications. For a thermometer of ex- 

 treme sensibility, mercury does not appear to be a convenient 

 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 

 observable 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 difference in the position of the head 

 of the column in the tube according to the manner in which 

 the glass is supported, and may therefore affect with uncer- 

 tainty the indications of the instrument. The former objec- 

 tion does not apply to the use of any fluid which perfectly 

 wets the glass; and the last-mentioned source of uncertainty 

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

 or greater expansibility by heat. Now the coefficient of ex- 

 pansion of sulphuric aether at 0° C. being, according to M. J. 

 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 ther- 

 mometer of much higher sensibility may be constructed with 

 aether than with mercury, without experiencing inconvenience 

 from the circumstances which have been alluded to. An 

 aether 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 cylin- 

 drical, and is about 3| inches long and fths of an inch in 

 diameter. The tube has a cylindrical bore about 6|- inches 

 long: about h\ inches of the tube are divided into 220 equal 

 parts. The thermometer is entirely inclosed, and hermeti- 

 cally sealed in a giass tube, which is just large enough to 

 admit it freely f. On comparing the indications of this instru- 



* See Dixon on Heat, p. 72. 



f Following a suggestion made to me by Professor Forbes of Edinburgh, 

 I have in subsequent experiments with this thermometer, used it with 

 enough of mercury introduced into the tube in which it is hermetically 

 sealed to entirely cover its bulb; as I found that, without this, if the ex- 

 periment was conducted in a warm room, the indications of the thermo- 

 meter were frequently deranged by the portion of the water which was left 

 free from ice becoming slightly elevated in temperature. 



