638 FREDERICK GUTHRIE ON THE THERMAL RESISTANCE OF LIQUIDS. 
§ 6. Rate of solidification as affected by temperature and pressure, and the presence 
of foreign bodies which may subsequently be entirely withdrawn, are instances in point 
in “ dead ” matter ; while the structures of plants and animals are so inconstant that 
every two specimens of the same organized body differ appreciably from one another 
even in texture. No solid substances are homogeneous. Metals all show irregular 
crystallization. The hardest gems have cavities. There are interstices between parts 
of a crystal which are in optical contact. In short, no sooner does a gas or liquid 
assume the solid form than it ceases to be homogeneous. 
§ 7. But under like external circumstances, two equal volumes of the same liquid are 
identical. No examination can distinguish the one from the other. The only appre- 
ciable difference, the only individuality is that of place. 
§ 8. One of the chief difficulties to be overcome in studying the thermal resistances 
of liquids meets us on the threshold, and arises from the expansion of liquids by heat. 
If the bottom, sides, or interior of a liquid mass be heated, the whole liquid is gradually 
set in motion owing to the expansion of the heated portions ; and the heat-effects 
observed at other places in the mass are chiefly due to the convection of heat by the 
moving liquid. 
§ 9. To get rid of such currents two methods are possible, (1) to heat the upper 
surface of the liquid, (2) to cool the lower one. For hy either of these means, the 
hotter portions of the liquid always rest above the cooler ones, so that no currents can 
be produced. The two methods should give equally correct and, indeed, in many cases, 
identical results ; in all cases results easily comparable. The choice is a matter of con- 
venience. I have preferred the first, because it is more easy to control exact and constant 
temperatures of any considerable range when above, than when below the atmospheric 
temperature. 
§ 10. It is clear that when a mass of liquid contained in a vessel is heated from above, 
the heat will be communicated to the containing vessel. If the latter conduct heat 
better than the liquid, heat will travel down it more rapidly than down the liquid, and 
the result will be the establishment of currents such as are shown in Plate LXY. fig. 1 a. 
If the vessel be a worse conductor than the liquid, it will withdraw from the liquid in 
contact with it a portion of the heat which has been conducted from above ; so that 
again currents will be established as in Plate LXV. fig. 1 b. In both cases, therefore, 
the apparent conduction of the heat by the liquid will be modified by the different con- 
ducting-power of the vessel. But as the mass of the liquid varies as the cube, while 
the surface of the vessel varies only as the square of the linear dimension, the disturbing 
influence of the vessel’s walls may be diminished to any amount by increasing its size. 
Vessels of great size can, however, only be employed for a few liquids. 
II. Sketch of former Experiments. 
§ 11. The experiments which have been hitherto performed on the conduction of 
heat through liquids are briefly the following : — 
