642 FREDERICK GUTHRIE ON THE THERMAL RESISTANCE OF LIQUIDS. 
The lower cone and tube form in fact an air-thermometer, the liquid in the stem of 
which may be adjusted to any height. 
§ 23. The platinum surfaces of the cone bases are ground optically flat and polished*. 
When breathed upon and brought into contact, they fit one another so exactly that the 
one may be almost lifted by the other, owing to their adhesion. Before being used, 
they are washed in succession with hot caustic soda, nitric acid, alcohol, and water. 
§ 24. It is clear that if the micrometer-screw is read when the cones are in actual 
contact, and again when they have been removed to some considerable distance (say 10 
millims.), we know how much the screw must be turned to give a motion of 1 millim. 
In my apparatus TOG turn of the screw corresponds to 1 millim., and as the screw-head 
is divided into 100 parts, it is easy to adjust the distance of the platinum faces from one 
another to 0*005 millim. The platinum faces are brought into an exactly horizontal 
position by placing a spirit-level on the lower one, and adjusting by means of the screws 
T, T. A thick ring of vulcanized caoutchouc (X) presses the tripod stand to the board. 
§ 25. The upper cone is then brought down to the required distance from the lower 
one, and the flat cylindrical interval between the two is filled by means of a pipette 
with the liquid through which the heat has to pass. When the distance (<$) between the 
faces is small (say 0*5 millim. to 1 millim.), the edge of the liquid is sensibly vertical, 
and the eye judges very accurately whether the interval is underfilled or overfilled. 
But at greater distances (from 2 millims. to 12 millims.) the lower portion of the liquid 
disk bulges out in consequence of the weight of the liquid, although both upper and 
lower face remain perfectly wetted. Thus at a distance of 6‘6 millims., water shows an 
appearance as in fig. 4, Plate LXY. The mere filling of the interval as full as it will hold 
is then a very imperfect guide. In such cases small strangulated pipettes are used 
(fig, 5, Plate LXY.), which deliver a volume of liquid exactly equal to the volume of the 
interval when the faces are 0 - 5 millim. or TO millim. apart. By this means, although 
the edge of the liquid be curved, we at all events know that the volume of the liquid is 
the same as if it were vertical and straight^. 
§ 26. Let us imagine that a known thickness of liquid is between the cones, that 
the capacity of B is known, that the tube Q has been calibrated, that the temperature 
of the air has been ascertained, that the height of the liquid in Q is noted, that the 
barometric pressure has been observed, and that the height of the column of water 
above the surface in the reservoir has been also measured. If now a current of hot 
water of known temperature is led through the tube C, the lower surface of A at once 
assumes and is continually maintained at a known temperature. The heat gradually 
* Held obliquely, the image of a straight line seen on the base of the cone coincides with a straight edge, 
however the cone may be turned on its axis. 
t It is noteworthy how remarkably the cohesive preeminence of water appears in these experiments. Other 
liquids can be obtained in disks of 4 millims. to 5 millims. only with great care : water may be got in disks of 
8 millims., or even 12 millims. It will be remembered that I have elsewhere shown that of all liquids water has 
the greatest drop-size (Proceedings, Royal Society, 1864). This fact is also to be attributed to the superiority 
of that liquid in cohesion. 
