Density, and Temperature of Gases. 167 



uniform temperature, the rule being that no change was per- 

 ceived in five minutes' time in the interior thermometer. 



Previous to commencing the experiments, the calibre of the 

 tube of the mercury gauge was found by measuring the lengths 

 of it filled by 100 grains of mercury ; and the heights of the 

 mercurial column at different places above the level in the 

 receiver by measurements of the heights occupied by successive 

 1000 grains, from 1 lb. avoirdupois up to 1 lb. plus 7000 grains; 

 and then a formula was obtained giving the height of the mer- 

 curial column belonging to any given reading on the scale of the 

 gauge. Care was also taken to compare the readings of the 

 interior and exterior thermometers in their places as actually 

 used, and after long rest, when the room where the trials were 

 made was only undergoing small changes of temperature. 



The experiments made in March showed that the difference 

 of the temperatures of the air in the jet and in the receiver, 

 when the mercurial gauge stood about 60 inches, could not be 

 near so much as 8° Fahrenheit, as the mercury of the exterior 

 thermometer rose (at the second impression) rapidly, when the 

 jet passed into the chamber containing its bulb. In these expe- 

 riments, the nozle with the thermometer in its chamber attached 

 were all cooled together to the lower temperature, and screwed 

 rapidly in their place, with a pair of leather gloves on the hands, 

 to get the full impression of cold when the stopcock was 

 opened. 



In June, when some of the precautions before mentioned had 

 been adopted, the experiments were recommenced with the 

 exterior thermometer and its chamber cooled 13° below the air 

 in the receiver ; and with this difference, and various interme- 

 diate ones up to a difference of 3°, the thermometer rose rapidly 

 when the jet passed into its chamber with a rarefaction two, or 

 when the air passed from the pressure of three atmospheres to 

 the pressure of the external atmosphere. When the difference 

 was only 1°, there was always a small but distinct fall of the 

 thermometer on the jet passing into the chamber; when the 

 differences were 1°'5 to 2°, irregularities in the results appeared, 

 until the final precaution was adopted of passing the jet the 

 moment the exterior thermometer was put in its place, and this 

 required experience with the microscopic adjustments. 



Experiments were also tried with a rarefaction about unity, or 

 with the mercurial gauge indicating an additional pressure of 

 30 inches of mercury upon the air in the receiver. The effects 

 were quite visible, though slower than in those before mentioned, 

 on account of the less rapidity with which the air in the chamber 

 was replaced. It was found that the temperature of the jet 

 could not be nearly 1° below that of the air in the receiver. 



