344 MM. Jamin and Richard on the Cooling of Gases. 

 of h; we shall therefore have 



pJ-^=sKf(R,h) (2) 



If, theii; we knew/(tl, h) — that is to say, if we knew the law 

 according to which the velocity of cooling -j— of a heated gas 



varies^ -j- could be calculated by means of equation (1); and we 



could measure directly and without hypothesis the cooling-power 

 of the gas. The two questions are connected and equally inter- 

 esting. We shall therefore divide this memoir into two parts, 

 and, 1st, study the cooling of heated gases within an enclosure; 

 2ndly, measure the heat given up to these same gases by a heated 

 solid placed in their midst. 



Part I. Laws of the Cooling of Gases. 



The apparatus consists of a large balloon of glass, 32 centims. 

 in diameter. It is immersed in a vat at degrees filled with 

 water which is continually agitated by a current of air, and con- 

 nected with a mercurial manometer, which, observed by means 

 of a cathetometer, gives the initial pressure H. The balloon is 

 accompanied by a bottle, which shares its temperature; the 

 two communicate by a three-way cock, through which a vacuum 

 can be produced in both or a gas introduced at the same pres- 

 sure. When this is done, the communication is closed, and a 

 second is opened through a differential vrater-manometer, one 

 of the two branches of which communicates with the bottle, the 

 other with the balloon. The heights are equal when the tempe- 

 ratures are the same; but if we heat the gas in the balloon, it 

 assumes an excess of pressure h, which the manometer measures 

 with great delicacy, since it contains water, whatever the initial 

 pressure H. 



The balloon is traversed along one of its horizontal diameters 

 by a platinum spiral of great resistance ; this is a focus, of neg- 

 hgible mass, which developes, by means of an electric current, 

 a known quantity of heat, of which one part traverses the enclo- 

 sure by radiation, while the other heats the gas. The wire is 

 heated to redness; and when the manometer has attained its 

 stationary condition, the circuit is broken. The spiral is extin- 

 guished immediately ; at the end of ten seconds it is entirely 

 cooled; and from that moment h diminishes regularly with de- 

 creasing rapidity. Then, while an assistant counts the time 

 aloud, at the end of every five seconds the observer reads the 

 value of h, which is written down by a third person. Use 

 readily familiarizes this kind of observation ; and more precision 



