THERMOMETER AND PYROMETER. 



ened with any very evaporable fluid, 

 such as aether or alcohol, and a stream 

 of air blown through the tube, a more 

 rapid and more conspicuous deposition 

 of dew takes place on the surface of the 

 metallic tube, than we ever recollect to 

 have observed in similar experiments. 



Fig. 50 is Dr. Gumming' s hygrometer 

 fitted to a portable air-syringe, by which 

 a current of air is produced through 

 the tube B B. The bulb of the delicate 

 thermometer within it is surrounded 

 with fine sponge, to retain the evapo- 

 rable fluid ; the tube B B is of highly 

 polished metal, with an aperture in its 

 upper part covered with a glass tube, 

 for the inspection of the thermometer, 

 as represented in the figure. 



Fig. 50. 



CHAPTER VI. 



On the Imperfections common to all 

 Instruments for the Indication of 

 Heat. 



1. THE terms thermometer and pyro- 

 meter might lead to the supposition, 

 that the instruments so designated were 

 actual indicators of the quantity of ca- 



loric contained in those bodies to which 

 they are applied ; but a single experi- 

 ment is sufficient to show that this view 

 is erroneous. If we place equal quan- 

 tities of water and of snow, both at 

 temperature 32, in a room at 60, the 

 temperature of the water will, as indi- 

 cated by the thermometer, after some 

 time, rise considerably ; but the effect 

 of the heat on the ice will only be to 

 melt it partially, while its temperature 

 remains steadily at 32. Here we have 

 caloric received by the ice which does 

 not affect the thermometer. 



The principle upon which the ther- 

 mometer and the pyrometer act is, the 

 tendency which heat or caloric has to 

 diffuse itself among contiguous bodies. 

 When applied to a hot body, they ac- 

 quire a portion of the heat from that 

 body ; and when applied to a cold one, 

 they communicate to that body a por- 

 tion of their own caloric. These changes 

 in the quantity of its own caloric are in- 

 dicated by changes in the bulk of the 

 thermometric fluid, orpyrometric piece; 

 and such instruments, therefore, do 

 no more than show a certain excess 

 of heat given out by the hottest to the 

 coldest body. On this ground the 

 names of thermoscope and pyroscope 

 are more suitable for such instruments 

 than their more common designations. 



That different bodies, in equal quan- 

 tities, whether measured by weight or 

 volume, contain unequal quantities of 

 caloric, has been established by the 

 investigations of Boerhaave, Black, 

 Wilcke, Irvine, Crawford, Lavoisier, &c. 

 It does not belong to this place to enter 

 into this subject, but it is sufficient to 

 mention the grounds for this important 

 conclusion. 



If we mix one pound of water at 

 212, and as much water at 32, when 

 due precautions are employed to mix 

 them without loss of heat or the addi- 

 tion of extraneous temperature, the ther- 

 mometer plunged in the mixture will 

 indicate very nearly 122, or the arith- 

 metical mean between the extremes ; 

 which proves that equal quantities of 

 the same body contain quantities of calo- 

 ric proportional to their temperature. If, 

 however, we mix a pound of mercury 

 with a pound of water, at different tem- 

 peratures, when the mercuiy is the hot- 

 test, the temperature of the mixture will 

 be greatly below the mean ; and when 

 the water is the hottest body, the mix- 

 ture will be greatly above the mean tem- 

 perature. A series of such experi- 

 E 2 



