640 FREDERICK GUTHRIE ON THE THERMAL RESISTANCE OF LIQUIDS. 
between thirty and sixty hours the thermometers became stationary, and from the degrees 
of heat which they marked, the experimenter concluded that the temperatures (above 
the air) were in geometrical progression (the intervals being equal), and that accordingly 
the law of conduction for water is the same as that for solids. M. Despretz found that 
the conductivity of salt water is sensibly the same as that of fresh. Measured in this 
way, it appeared that water had a conductivity about that of copper. 
§ 16. It was found in these experiments that the axis of the cylinder was hotter than 
the parts towards the circumference, and that the interior of the wall of the wooden 
cylinder was cooler than the surface in contact with the liquid. These circumstances 
appear to detract somewhat from the value of M. Despretz’s results, because the exist- 
ence of inequality of temperature invariably points to the existence of vertical currents. 
Granted that the wood conducts worse than the water, what is the consequence % Not 
that the wood is of no influence, for a portion of the heat of the water must necessarily 
be. given to the wall which it touches, albeit more may be given to the water below, so 
that currents arise according to § 10, Plate LXV. fig. 1 b. 
§ 17. Another source of error, as it appears to me, in these experiments, lies in the 
intermittent manner in which the hot water is renewed in the source of heat. The time 
required to produce a given effect must certainly be thereby very materially increased. 
Tor when a quantity of hot water is poured in and allowed to stand, not only is the top 
of the conducting column heated but the bottom of the heating mass is cooled, the two 
quickly assume nearly the same temperature, which is below that of the water originally 
added. When, therefore, a fresh supply of hot water arrives, its heat has to traverse a 
series of strata of water, each of which is at a lower temperature than it would have 
been if the supply of hot water had been continuous, — that is, if the upper layer of the 
column had been maintained at a maximum temperature instead of being only moment- 
arily raised to that temperature by each successive addition of hot water to the upper 
vessel. 
§ 18. It seems, indeed, clear that the condition of M. Despretz’s column, after the 
thermometers have become stationary, results from the establishment of thermal equili- 
brium between the continual accession of heat from above downwards, and the escape 
of that heat through the sides of the containing vessel by radiation and atmospheric 
conduction ; otherwise it is impossible, with our present knowledge of the nature of heat, 
to explain the fact observed, that even after sixty hours the lowest thermometers show 
no increase of temperature in the lower strata of the column. 
III. Description and use of the Diathermometer. 
§ 19. The object immediately proposed in the following investigation was to deter- 
mine the relative and absolute thermal resistance offered by a given thickness of a given 
liquid in a given time, when the liquid has a given temperature, and when the tempe- 
rature difference between the liquid and the source of heat is known. To effect this, an 
apparatus had to be devised which would be applicable to many different liquids, and 
