barus.j VISCOSITY OF GASES. 245 
least for empirical temperature measurement, beyond a doubt. In later 
experiments the proximity of the burner n to the receiver B was found 
objectionable, and the cramped arrangement of this part of the appa- 
ratus interfered in other ways with, accurate measurement. Retaining 
the essential features of Fig. 43 the apparatus was modified as follows: 
In Fig. 44, B B is the receiver into which the mercury flows on pass- 
ing out of the Mariotte flask. The lettering of this figure is in con- 
formity with that of Fig. 43, but the disposition chosen is such as to 
show additional parts. The manometer R B' B' is here very clearly 
given, quite filled with mercury, as is the case at the outset of the ex- 
periment. It will be seen that as the pressure in B increases, the mer- 
cury in the reservoir B' B' passes. into the tube P, leaving a vacant 
space in B' B' above the lower meniscus. But as the experiment pro- 
ceeds, and the receiver B fills with mercury, this metal eventually falls 
into the communication tube r, while the air displaced escapes into B 
through the capillary tube s. Hence each time B is filled with mercury 
a fixed volume of air must pass out of it, and the manometer BB' B' in- 
troduces no discrepancy. The cock D being closed, mercury falls into 
B from Q, as the column passes through the lateral tube P TJQ< The top 
of B'B 1 is cut off obliquely, so as to guide the descending drops or little 
stream of mercury at once into B. The drying-tube of the apparatus 
communicates with w, and may be shut off by the faucet S. In this 
way dry air or any other gas may be easily introduced. The capillary 
tube is here placed at some distance from B and in connection with the 
lateral tube k. A small sensitive thermometer sealed in the vertical 
tube T indicates the temperature of the gas as it escapes into the 
capillaries. For the sake of clearness in diagram the tubes $, P, T 
are represented as placed in a single vertical plane. In practice the 
rubber cork is perforated symmetrically and larger tubes may be chosen. 
The tube P Q comes apart at U, and hence the tubes and the manometer 
may be easily withdrawn from B, When in use it is necessary to seal 
in the cork and the divers glass tubes with resinous cement. 
It is easily seen that after B is full of mercury the Mariotte flask A 
may be lowered, and on opening the stop-cocks D and 8 mercury will 
flow back from B to A. The receiver B is thus filled with dry gas and 
again ready for experiment. But the order of manipulation is import- 
ant, and will be indicated in connection with the data given below. 
In using the method of compression preferably to a method of ex- 
haustion, by both of which a flow of gas through the tubes can be 
secured, I was guided by the belief that the methods of measurement 
in the former case are more easily capable of variation. This the present 
chapter may show. Again, the tendency of the dissociated hydrocar- 
bon gases of the burner to permeate the walls of the platinum capillary 
tubes at high temperatures, is of less disturbing effect wiien the current 
of gas is condensed than when it is ratified. 
The capillary apparatus. — In Fig. 44 there is ample room for air to 
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