488 
POPULAR SCIENCE EE VIEW. 
the purposes of' her callings may yet be able to enter the latter. 
We might multiply illustrations of this kind to an indefinite 
extent, but the above are sufficient as an introduction to the 
general proposition which we would here lay down ; viz., That 
•whereas all bodies whatsoever expand by heat and contract by 
cold , thermometry is the art of applying the known expansion 
and contraction of one body , in such a way that it may serve as 
a standard of reference with which to compare the expansion and 
contraction of other bodies. 
A subtle physicist might find fault with this enunciation; 
a lawyer might say, You rush headlong into an assumption 
without adequate proof. We should reply to the latter, that 
our object is to apply facts, and not to argue; and we should 
hope to disarm the former by saying that it was our desire 
to divest the matter of rigid and dry technicalities. 
The same body always has the same volume at the same 
temperature, and always suffers the same change of volume 
with the same change of temperature. Since volume and 
change of volume admit of the most exact measurement, they 
become a very convenient means for determining the change 
which takes place in the temperature of any body. Although all 
bodies are susceptible of dilatation and contraction by changes 
of temperature, yet all are not equally fitted for thermometrical 
purposes : for many reasons, some of which will presently be 
adverted to, mercury is found to be best adapted for the 
purpose. We shall therefore commence with a description 
of the simple mercurial thermometer. 
This instrument consists in its ordinary, or, as we may call 
it, its typical form, of a capillary glass tube from eight to 
fifteen inches long ; to one extremity of which (which, when 
mounted, becomes the lower extremity) a glass bulb is blown, 
the bulb and part of the tube being filled with mercury. 
When such a tube is exposed to an increase of temperature, 
the glass and the mercury contained in it will both expand. If 
these expanded in the same proportion, the capacity of the bulb 
and tube would be enlarged in the same proportion as 
the mercury in them, and consequently the level of the 
mercury in the tube would remain unaltered. If, however, 
the expansion of the bulb and tube be different from that 
of the liquid metal contained in them, the level of the 
column in the tube will, after expansion, stand higher 
or lower than before, according as the expansion of the 
mercury is greater or less than the expansion of the bulb 
and tube. 
It is found by experiment that the dilatability or expansive 
power of mercury is greater than that of glass in the proportion 
of nearly 20 to 1 ; and, consequently, the capacity of the bulb 
