21* On the Determination of the [Sept- 



§ II. — Apparatus employed to make a regular Current of hot 

 Gas pass through the Calorimeter. 



We employed, in order to procure an uniform purrent of gas, 

 a gazometer, which we believe was invented by Dr. Wollaston, 

 and which unites a great deal of simplicity with the greatest pre- 

 cision. A few words will explain the nature of this ingenious 

 instrument. Suppose a globular glass vessel, A, (Fig. 2,) filled 

 with water, and placed above a reservoir of glass or metal, B, 

 filled with any gas insoluble in water. Let these two vessels 

 communicate by a vertical tube, C D, which may be shut by a 

 stop-cock, E. Suppose, likewise, that the upper surface of the 

 water contained in A is G H. It is evident, that if we open the 

 stop-cock, E, the water will fall into the vessel, B, and drive out 

 the gas, which will make its escape by the mouth, L, (the stop- 

 cock, M, being open). It is equally obvious that the force with 

 which the water of the vessel, A, will run down, at first equal 

 to the column of water, H K, will diminish in proportion as the 

 surface of the water, G H, sinks. But then, if we shut exactly 

 the mouth, F, and establish the communication of the vessel, 



A, with the external air by the tube, N O, open at its two ends, 

 the ah", in order to introduce itself into the vessel, A, and fill 

 up the place of the water that has run out, must overcome the 

 pressure of the column of water, H I. Of course the water 

 will only run into the vessel, B, with a force equal to the 

 column H K, minus H I, that is to say, the column I K, which 

 is a constant quantity, as long as the surface of the water conti- 

 nues higher than O I. Now suppose the reservoir B be totally 

 emptied of gas, and filled with water from the vessel, A. Shut 

 the stop-cocks, E M, and introduce by the tube, Q R, which 

 goes to the bottom of the vessel, a constant current of gas, 

 coming from a similar gazometer. In these circumstances, if 

 we open the stop-cock, P, to let out the water from the reservoir, 



B, it is clear that this gas, in order to get into the reservoir, B, 

 is obliged to overcome a resistance represented by the column of 

 water which this reservoir contains. On the other hand, it is 

 attracted by an equal force, namely, the force with which this 

 water tends to run out by the stop-cock, P, and which is repre- 

 sented by the same column. These two forces being equal, and 

 opposite, it follows that the regularity of the gas entering by the 

 tube, Q R, will not be disturbed, and that the reservoir, B, 

 will be filled with gas coming from the other gazometer, with- 

 out having any effort to overcome. The stop-cock, E, remaining 

 shut during the whole of this operation, there will be time to 

 /open the mouth, F, and to fill the vessel, A, with water, in 

 order to recommence the operation. It is easy to see that with 



