BETWEEN THE VISCOSITY OF LIQUIDS AND THEIR CHEMICAL NATURE. 429 
instruments, were employed. The working range of the first instrument extended 
from 10 ° to 110 °; that of the second from 100 ° to 210 °. They were of the 
Geissler pattern, made of Jena glass, and had heen compared with standards at the 
Physikalisch-Technische Reichsanstalt at Charlottenburg. Each thermometer was 
provided with two fixed points and was divided into half-degrees and could be read 
by the telescope to hundredths. The position of the two fixed points was ascertained 
from time to time, and the necessary corrections introduced into the temperature 
readino-s. During- the time over which the observations extended there was no 
appreciable alteration in the capacity of the bulbs. 
In considering the influence of thermometric errors it must be remembered that 
cly^jclT varies for different liquids, and moreover for any one liquid varies greatly with 
the temperature. Thus for water the value of this rate of change is about 18 times 
as large at 0 ° as it is at 100 °. This example serves to illustrate the general rule that 
cmdT is much larger at low than at high temperatures. On the other hand, in con¬ 
sidering the effect of possible errors due to this circumstance, it must be remembered 
that it is much easier to keep the temperature constant at low temperatures, and that 
the accuracy of the final result is increased by the multiplication of thermometer 
readings at the low temperatures, and that the longer times of flow tend to ensure that 
the liquid has actually the temperature of the bath in which the whole is immersed. 
An error of 0 °’l in observing the temperature may be taken for an average value of 
the viscosity as corresponding to about 1 part in 1000 in the value of 17 . 
Determination of Pressure .—The pressure under which the liquid was driven 
through the capillary tube was measured by means of a water-manometer. This 
consisted of a glass U-tube fixed, as shown in fig. 2 , to a stout wooden frame, provided 
with a plummet and levelling screws. The shorter limb of the manometer was con¬ 
nected with the air-reservoir, and between the limbs was a thermometer to determine 
the temperature of the water. Two millimeter scales, each 10 centims. long, were 
etched on each limb, the middle points of the inner pair and of the outer pair being- 
equidistant from the middle point of the longer limb. The two upper scales were 
read from below upwards, the two under scales from above downwards. The pressure 
head was thus the sum of the scale-readings, ‘plus the distance between the zeros of 
the scales. The quantity of water was adjusted so that the levels were always on 
corresponding scales, i.e., either on both of the inner or on both of the outer scales. 
Pressure heads of circa 130 centims. were measured by the outer scales, and those of 
circa 100 centims. by the inner. The inner scales were principally used for such 
liquids as would, under the higher pressures corresponding to the outer scales, give 
times of flow under the three minutes limit. The lengths of the scales were tested 
and the distance between their zero-points measuied by means of a De La Hive 
cathetometer. As the pressure head was to be expressed finally in terras of water at 
4°, the scale of the manometer was corrected to 4°. This was done once and for nil 
from the cathetometer readings at the mean atmospheric temperature by applying a 
