Elastic Force } Density, and Temperature in Gases. 61 

 With the smallest receiver, the result 



and by Poisson's formula the result is 53° # 7. 



In another experiment with the smallest receiver the result 

 was 



*°-2' o =50°-5, 



for which case Poisson's formula gave 92°*4. 



Although the results at the smaller rarefactions were not very- 

 different from Poisson's rule, yet those at greater rarefactions 

 did not appear to accord with it. In this state of the experi- 

 ments I laid the results aside until lately, when I wished again 

 to try if any better method of using Breguet's thermometer could 

 be found, and more decisive results obtained. 



On placing Breguet's thermometer in the large receiver of 

 the air-pump, its rnyiacity being about 650 cubic inches, with an 

 opening of 2\ inches diameter at the top, to be closed by a cir- 

 cular plate of glass smeared with grease, I found that, having 

 exhausted the receiver to a given degree, then when the plate of 

 glass was slid away suddenly (say, in one tenth of a second) the 

 agitation of the air in the receiver caused the thermometer to 

 arrive at its maximum in one second of time, and therefore pro- 

 bably indicated the true result very nearly. It was thus desi- 

 rable to make the experiments with all possible accuracy, and 

 carefully allow for the capillary depression of the mercury in a 

 new barometer-gauge Experiments were also made by using a 

 separating diaphragm of tissue paper in a bag-form at the upper 

 part of the receiver, and held up by an elastic ring of wire press- 

 ing it to the glass. The diaphragm yielded to the re-entering air, 

 but kept it separate from the larger part of the rest. With the 

 small condensations the thermometer resumed its sluggishness, 

 arriving at its maximum only in four or five seconds ; but as 

 the condensation increased, it acted more rapidly, and at the 

 greatest condensation arrived at its maximum nearly as quickly 

 as without the diaphragm, and with nearly the same indication 

 of heat. I consequently conclude that the phenomenon of 

 quickness was not due to the re-entering air, otherwise than as 

 producing agitation and a rapid effect upon the helix of the 

 thermometer. Other experiments were tried with the bell-glass 

 of the thermometer left over the helix, but raised on pieces of 

 card, and others, again, with the bell-glass raised out of its 

 groove and moved sideways, leaving considerable space for the 

 air to pass. These gave the maxima results smaller than when 

 the helix was uncovered, but they occurred in one second of 

 time. 



