1813.] Specific Heat of the different Gases. 217 



§ HI. — Method of determining the Heat given out ly the Gas in 

 passing through the Calorimeter. 



In the description which we have given of our gazometers we 

 have explained the method employed to heat the gases. It is 

 obvious, that whether they reached the temperature of boiling 

 water, or remained a little colder, they would always acquire a 

 constant temperature. It was necessary to know this temperature 

 before they entered into the calorimeter. It must have been less 

 than that of boiling water; for it was not certain that it would 

 acquire that temperature in passing through the tube ; and even 

 if it had, as it had to pass through a small portion of the tube 

 not surrounded with steam, before getting into the calorimeter, 

 its temperature would necessarily sink somewhat in that place. 

 It seems, at first sight, that it would have been easy to determine 

 this temperature at the entrance of the gas into the calorimeter, 

 by means of a thermometer placed in that spot, in the middle of 

 the current; but we convinced ourselves, by experiments too 

 tedious to describe here, that a thermometer placed in such a 

 situation always stands lower than the true temperature of the 

 current. It will be easy to perceive that this must be the case, if we 

 reflect that bodies placed in an aeriform fluid, being influenced 

 as to their temperature by the surrounding bodies, assume a 

 middle temperature between that of these bodies and the aeri- 

 form fluid with which they are surrounded. In the present case, 

 the tube, in the centre of which the thermometer is placed, 

 being colder than the gas, must act upon the thermometer by 

 means of its radiating quality, and sink its temperature. We 

 ascertained, by experiment, that a thermometer with a gilt bulb 

 stood always higher in such a situation than a common thermo- 

 meter. 



This difficulty of appreciating the temperature of a current of 

 "as at its entrance into the calorimeter induced us to make the 

 tube between the vapour tube and the calorimeter as short as 

 possible. By this means we succeeded in having the tempera- 

 ture of the gas very little lower than that of boiling water; and as 

 we were certain that this temperature cpuld neither be lower 

 than that indicated by the thermometer, noi* higher than that of 

 boiling water, it is obvious that by taking the mean between 

 Be two temperatures wc could not commit an error of great 

 importance.* 



We did not experience the same difficulty in determining the 



temperature of the gases alter they came out of the calorimeter, 



\ thermometer placed at the end of the serpentine allowed us 



* The thermometer usually stood between lOO^ and 204' Fahrenheit. 



