EXPERIMENTAL DETAILS. 153 



The calorimetric measurements, carried out to within ^J of 

 a degree, involves a smaller error than by the old method, 

 seeing that the combustions are generally total, and the correc- 

 tions extremely reduced, through the short duration of the 

 experiment. 



However, the accuracy is limited by the weight of the 

 substance on which we are obliged to operate ; the weight of 

 the carbonic acid formed generally not exceeding 0*200 grms. to 

 0*300 in the most favourable cases. 



The quantity of gas burnt may be estimated either from its 

 initial volume, or from the weight of its products. 



The estimation of the initial volume presents great difficulties, 

 owing to the necessity of taking into account the internal spaces 

 of the tubes joining the bomb to the receivers in which the 

 gases are measured. It has, however, been effected in the case 

 of hydrogen. 



But in the majority of cases it is preferable to weigh, after 

 combustion, the gaseous products, which reduce themselves 

 ordinarily to carbonic acid. With this object the gases are 

 collected from the bomb, after explosion, by means of a mercury 

 pump, and passed through a tube filled with pumice-stone and 

 sulphuric acid, which dries them, then through a Liebig tube 

 filled with potash, followed by a TJ tube filled with solid potash, 

 in order to absorb the carbonic acid. The bomb is thrice 

 filled with air free from carbonic acid, in order to clear out the 

 gases completely, and each time the gases extracted from the 

 bomb are passed through the Liebig tube. The latter, and the 

 tube filled with solid potash, are finally weighed. It is necessary 

 to further make the following verifications. 



In the first place, the combustion of each gas is effected in 

 the eudiometer, over mercury, in order to see that it is pure and 

 gives the theoretical figures. 



Then a similar combustion is carried out in the calorimetric 

 bomb, the whole of the gases are extracted from it by the pump, 

 and collected over mercury. After the absorption of the car- 

 bonic acid and of the oxygen, it is ascertained whether there 

 remains any trace of combustible gas (carbonic oxide, hydrogen, 

 marsh gas, etc.). 



This verification is made, first with the aid of acid cuprous 

 chloride, then by means of a fresh attempt at burning, by a 

 proper quantity of oxygen. If nothing burn, there is added 

 to the mixture the half of its volume of electrolytic gas, and the 

 attempt is repeated. 



In this manner it has been ascertained that the combustions 

 are total with all hydrocarbon gases properly so called, such as 

 methane, methene, ethylene, ethene, ethane, dimethyl, pro- 

 pylene, etc. 



8. The combustion of nitrated, chlorinated, brominated, iodated 



