of the Theory of Heat. 269 



city with which the heat is dissipated. However, by uniting the 

 different causes which contribute to retard the coohng of a given 

 mass, we are enabled, as our experiments have proved, to place 

 it in such circumstances that the differences in tlie conductibiHty 

 of the substances operated upon, have no sensible influence on 

 the measure of the capacities. 



To attain this end, the most obvious method is not to begin the 

 observation till the temperature of the body is only a few degrees 

 higher than that of the surrounding bodies. All our experiments, 

 therefore, were made in temperatures between 10"^ and 15° (cen- 

 tigrade) above the surrounding medium. The changes of tem- 

 perature should be measured with the greatest care; for even a 

 slight error in the estimate might occasion a great one in there- 

 suit. By operating, as we have stated, at the same temperature 

 for all the bodies, we avoid errors resulting from the graduation 

 of the thermometer ; and by observing this instrument through 

 a lens, we can increase the size of the degrees so much as not 

 to commit an error exceeding the 50th of a degree, — a quantity 

 so minute that it may be disregarded. To obtain uniformity of 

 temperature in the ambient medium during the whole time of 

 every experiment, the body was always placed in a vessel whose 

 sides were blackened interiorly, and covered on all parts with a 

 thick coating of melting ice. 



To this first means for diminishing the rate of cooling we added 

 another, the influence of which we could calculate from our 

 knowledge of the laws of the communication of heat. From these 

 laws it results that the velocity of cooling of a body may, cceleris 

 paribus, be considerably diminished when its surface possesses 

 but a very weak radiating power, and is immersed in an air very 

 much dilated. To accomplish this, we determined to operate 

 on solid bodies only in a state of very fine powder. In this state 

 they were strongly pressed into a cylindrical vessel of silver, very 

 thin, very small, and the axis of which was occupied bv the bulb 

 of the thermometer. This silver cylinder was then placed in the 

 centre of the vessel, the air contained in uhich was rarefied till 

 its tension did not exceed two millimetres; and care was taken 

 to reproduce the same rarefaction in each experiment. 



We thus succeeded in making the cooling of very small bodies 

 go on very slowly, and consequently easv to be observed with pre- 

 cision. It is sufficient to say, that when measuring the capacities 

 of the densest bodies, as gold and platinum, the (|uantities on 

 which we operated did not exceed ^K) grammes; and that the 

 time of cooling was never less than 15 luinutcs. 



We ought now to give the formula which served for the cal- 

 culation, but the details would lead us into a discussion which 

 we reserve for the publication of the definitive results of all the 



direct 



