744 
PROFESSOR 0. REYNOLDS ON CERTAIN DIMENSIONAL 
The convergence of these results is best seen in the accompanying diagram. The 
curves are drawn through points of which the pressures are abscissae, and the differences 
of pressure (on a different scale) are the ordinates. 
The maximum differ ence of pressure (carbonic acid). 
22. The curves, fig. 3, show that the differences both for hydrogen and air appear 
to be tending, as the pressure rises, to a maximum value. This was exactly what was 
expected from Law III., Art. 9, and had the apparatus been capable of withstanding 
considerable pressures it would have been desirable to have raised the pressure until the 
maximum was passed. But it appeared that the same end might be more readily 
accomplished in other ways. 
Owing to the great density and low coefficient of diffusion of carbonic acid, it seemed 
to be probable that with this gas the difference of pressure would reach a maximum at 
considerably lower pressures than either hydrogen or air. Carbonic acid was therefore 
tried. 
Table V.—Thermal transpiration of carbonic acid by meerschaum plate No. 2 
('25 inch or 6'3 millims. thick). Temperature of steam, 212 c F. or 100° C.; tem¬ 
perature of water, 47° F. or 8° C. 
Mean pressure by 
gauge. 
vacuum 
Difference 
by siphon gau 
of pressure 
ge, March 10. 
Ratio of mean pressure to 
difference of pressure. 
inches. 
millims. 
inch. 
millims. 
30-1 
764 - 5 
•13 
3-302 
230 
19-5 
495-3 
•16 
4-064 
122 
14-25 
361-8 
•16 
4-064 
89 
10-5 
266-7 
•15 
3-810 
70 
8-0 
203-2 
•13 
3-310 
61 
4-5 
114-3 
11 
2-794 
40 
2-0 
50-8 
-08 
2-032 
25 
1-0 
25-4 
"05 
1-270 
20 
0-5 
12-7 
•04 
1016 
12 
Table V. shows that with carbonic acid the maximum difference was at a pressure 
between 20 and 15 inches, the difference rising as the pressure fell from 30 inches to 
this point. After this point was passed the difference fell with the pressure. 
The curve on fig. 3 which represents this table shows the point of maximum 
difference, and the figure also shows that as the pressures became small the curve for 
carbonic acid converges towards the curves for air and hydrogen. 
These results for carbonic acid are perhaps sufficient to verify Law III. respecting the 
existence of a maximum. But they were obtained with considerable trouble, as the 
india-rubber tubing absorbed the carbonic acid very rapidly, and so caused considerable 
disturbance. For this reason carbonic acid was not- again used. 
