-------------+----------------+---------------------+
| | Resistivity | |
| Substance. | in Megohms | Observer. |
| | per c.c. | |
+---------------------------+----------------+---------------------+
| Ethyl alcohol | 0.5 | Pfeiffer. |
| Ethyl ether | 1.175 to 3.760 | W. Kohlrausch. |
| Benzene | 4.700 | |
| Absolutely pure water | 25.0 at 18 deg.| Value estimated |
| approximates probably to | C. | by F. Kohlrausch |
| | | and A. Heydweiler. |
| All very dilute aqueous | 1.00 at 18 deg.| From results by |
| salt solutions having a | C. | F. Kohlrausch |
| concentration of about | | and others. |
| 0.00001 of an equivalent | | |
| gramme molecule[10] per | | |
| litre approximate to | | |
+---------------------------+----------------+---------------------+

The resistivity of all those substances which are generally called
dielectrics or insulators is also so high that it can only be
appropriately expressed in millions of megohms per centimetre-cube, or
in megohms per quadrant-cube, the quadrant being a cube the side of
which is 10^9 cms. (see Table VIII.).

Effects of Heat.--Temperature affects the resistivity of these different
classes of conductors in different ways. In all cases, so far as is yet
known, the resistivity of a pure metal is increased if its temperature
is raised, and decreased if the temperature is lowered, so that if it
could be brought to the absolute zero of temperature (-273 deg. C.) its
resistivity would be reduced to a very small fraction of its resistance
at ordinary temperatures. With metallic alloys, however, rise of
temperature does not always increase resistivity: it sometimes
diminishes it, so that many alloys are known which have a maximum
resistivity corresponding to a certain temperature, and at or near this
point they vary very little in resistance with temperature. Such alloys
have, therefore, a negative temperature-variation of resistance at and
above fixed temperatures. Prominent amongst these metal