736 
PROFESSOR O. REYNOLDS ON CERTAIN DIMENSIONAL 
The material first used for the plates was W edgewood biscuit ware, ^ths inch thick; 
and it was with this material after a long series of trials that connected results were 
first obtained. These results were very minute. With air at the pressure of the 
atmosphere, the greatest difference of pressure was T of an inch (2'5 millims.) of 
mercury. 
Having however once obtained this result, it was seen to follow from Law V., Art. 9, 
that greater results could be obtained with a finer plate. My idea was to try graphite, 
such as that used by Graham; but in the meantime it occurred to me to try meer¬ 
schaum, which proved to be a most convenient material, as it could be obtained in 
any sizes and readily cut into plates of any thickness. 
With this material, first used on March 7, the later results were very striking; the 
difference of pressure amounting to '25 of an inch with air at the pressure of the 
atmosphere, and to nearly an inch with hydrogen at the same pressure. 
The description of the details of the earlier experiments, together with the various 
difficulties which were met with and the means employed to overcome them, would 
take too much room to admit of their being given at length. I shall therefore proceed 
at once to the description of the apparatus in its final form, and shall confine myself to 
noticing only such results as are important to the subject. 
Description of the apparatus. 
11. This consisted principally of an instrument which may be called a thermo 
diffusiometer. 
This instrument, as shown in fig. la, Plate 47, consists essentially of two chambers 
separated by a plate of porous material, means being provided for keeping the 
chambers at constant but different temperatures for many hours at a time; also for 
measuring the pressure of gas in the chambers, for exhausting the chambers, and for 
bringing the chambers into direct communication when desired. 
The chambers are formed by tin plates separated by rings of india-rubber, between 
which is held the porous plate. The external diameter of the rings is about 3^ inches; 
and the internal diameter, the diameter of the chambers, is 1^ inches. The thickness 
of the rings, the depth of the chamber, is about -j^-ths of an inch. The porous plates 
are 2 inches in diameter, so that the edges are well covered by the rings of india-rubber 
which bound the chambers; and outside the plate is fitted another ring of india- 
rubber of the same thickness as the plate, so as to prevent any leak through the edges 
of the plate. 
Outside the tin plates which form the walls of the chamber, other chambers are 
formed in the same manner by rings of india-rubber and tin plates. These second 
chambers afford the means of regulating the temperature, steam being continually 
passed through the one and cold water through the other. The chambers are made 
air-tight by means of pressure, which is brought to bear by means of a wooden press 
into which the rings and plates fit. 
