which are evolved in Coal^Minea. 323 



fourth not all, however often (10 to 12 times) the process 

 might be repeated. 



In these experiments, there was no difficulty in excluding 

 the atmospheric air in the tubes to a minimum,* for it was 

 only necessary to repeat frequently the flowing through of the 

 pit-gas. But it is very difficult to determine directly the vo^ 

 lume of the tubes, viz. those tubes which are filled with chlo- 

 ride of calcium. This determination can, however, be made 

 by an indirect method. Thus, if we perform the experiment 

 twice, but with unequal quantities of gas, and if we measure 

 each time the increase of volume, we can thence determine 

 the quantity of gas which was contained in the tubes. Let 

 a and a! be these two portions of gas which have been mea- 

 sured in one of the two gasometers, and let b and U be the 

 additions to the volumes after the decomposition of the gas 

 in the heated porcelain tube, then, if x indicate the volume of 

 the tubes, and should the increase of volume in the two expe- 

 riments have taken place under the same circumstances : 



X -\-a'.h — x + a' : b', 

 ,1 ha' — 6'a 



consequently .... x=z ^, __^ — « 



I have performed several experiments, but soon found that 

 the increase of volume had not always taken place in perfectly 

 similar circumstances, whether it was that there had been a 

 variation in the heating of the porcelain tube, or that the car- 

 bon remaining behind from previous experiments exercised 

 influence over the decomposition. Thus, I did not in each 

 new experiment take the apparatus to pieces, and remove 

 the deposited carbon, because the volume of the latter was 

 too small to affect that of the porcelain tube in a sensible 

 manner. Out of many experiments, I select two which pre- 

 sented the greatest agreement. The measured volumes, re- 

 duced to 28 inches barometer, and 32° F. were the following : 



a = 42.97 vol. d = 8.58 



a' =43.71 ^'=8.69 



Hence z = 14.75 vol , and the increase of the volume amounts 

 to 0.149 of the original volume. 



* The presence of the atmospheric air would naturally have caused a par- 

 tial ignition of the inflammable gae in the heated porcelain tube. 



