Dr. F. Fox's Capillary Thermometer. 71 



DR. F. FOX'S CAPILLARY THERMOMETER. 



The subjoined notice of this instrument is derived from 

 " Notes," lately published at Derby, of Mr. D. Fox's Lectures 

 on Anatomy and Chemistry delivered at the Mechanics' In- 

 stitution in that town. We have not seen the capillary thermo- 

 meter, nor have we received information respecting it from any 

 other source. 



"This instrument will detect a variation in temperature equal 

 to the thirtieth part of a degree of Fahrenheit. The length 

 of a degree on the scale of a common thermometer seldom 

 exceeds one-eighth of an inch, but on this instrument the de- 

 grees are more than an inch in extent; and any person con- 

 versant with small measurements must know how visible the 

 twentieth or thirtieth part of an inch is to the naked eye. To 

 give an idea of the delicacy of the capillary thermometer, the 

 lecturer stated, that eight drops of water, at the heat of 200 

 degrees of Fahrenheit, were dropped into a pint of cold water (at 

 60°) when the quicksilver instantly rose one-eighth of an inch. 

 Here the portion of hot water was only a fifteen-thousandth 

 part, in proportion to the cold water to which it was added. 

 Dr. Francis Fox, in making some extremely delicate experi- 

 ments upon heat, found that the most minute instruments ca- 

 pable of being filled in the usual way with mercury, fell very 

 far short of the accuracy required by him. Notwithstanding, 

 therefore, the great difficulty of filling thermometers of very 

 fine calibres, particularly where the tube is excessively minute, 

 as in the present instance, the Doctor succeeded in construct- 

 ing the extraordinary and beautiful instrument then exhibited 

 by the lecturer. The method pursued was this : A bulb, or 

 ball, was blown at one end of a common thermometer-tube. 

 This was filled in the usual way, by applying heat to the 

 bulb, on which the atmosphere forces the mercury into the 

 partial vacuum within the ball. This done, the tube was made 

 red hot in the flame of the blowpipe, and drawn out into a 

 fine capillary tube : this may be finer or larger, according to 

 the intended delicacy of the instrument. During this opera- 

 tion, the capillary tube contained no mercury, which remained 

 in the lower part of the original thermometer-tube, occupying 

 also the whole of the bulb. A small piece of writing-paper 

 was then tied round the extremity of the tube, so as to form 

 a cavity, into which a little mercury is poured. The tube thus 

 prepared is suspended by the upper end with the fingers ; and 

 the mercury in the ball being very gradually expanded by 

 heat, in a short time rises up and fills the whole tube, until 

 it comes in contact with the mercury in the paper cavity at 

 the top. Then, if the instrument be allowed to cool, the mer- 

 cury 



