PROPERTIES OF MATTER TR THE GASEOUS STATE. 
763 
of some importance, and as the instant at which the eye reached the chronometer did 
not always correspond with the complete second or half second, there was a liability 
to this error; but this was to some extent obviated by making successive experiments 
for such small differences of pressure, that the differences in the reading were much 
less than a second, and passing over all the observations except those which corre¬ 
sponded with the beat of the chronometer. 
With the stucco plates, both for air and hydrogen, three series of readings were 
taken, and the agreement was found to be very close. 
With the meerschaum, the interval of transpiration was so long, about 12 minutes 
for air and about 3 minutes for hydrogen, that one series of experiments was considered 
to be sufficient. 
It is important to notice here, that while making these experiments I had not the 
least idea as to how the results would come out when they came to be compared. This 
comparison was not made for several weeks, as the logarithmic method of comparing 
them had not occurred to me at the time the experiments were made. 
The very remarkable agreement which has been found in the results cannot, there¬ 
fore, be owing to any bias in my mind, but must be entirely attributed to the accuracy 
of the means of observation. 
Purity of gases. 
38. The greatest care was taken to get the gas pure and dry. And as it had been 
found in the previous experiments that when the pressure in the instrument was low, 
the gas, particularly the hydrogen, was liable to become contaminated by infusion 
through the india-rubber, the experiments w r ere not continued to very low pressures 
and were made as rapidly as possible. 
The results of the experiments. 
39. Two plates w 7 ere tried, meerschaum No. 3 and stucco No. 2, which were both 
in their respective diffusiometrts just as they had been used for thermal transpiration. 
The results are given in the following tables :— 
