428 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The comparison of these last two tables with that concerning hydrogen 

 is instructive and valuable even if a small undetected constant error 

 is concealed in them all, — for such an error must be the same in each, 

 and thus eliminate itself during comparison. Evidently the coefBcieut 

 of expansion of nitrogen under constant pressure must be very nearly 

 the same as that of hydrogen, while that of carbon dioxide is far greater. 

 The newly found value 0.003727 for carbon dioxide is larger than the 

 value found by Regnaiilt (0.00371) ; but it must be remembered that the 

 latter is merely an average value over a wide range of temperature, 

 while the former involves only the change from 0° to 32.38°, where 

 carbon dioxide is rather a vapor than a gas. The highest value found 

 in the first experiment is probably to be attributed to the adsorption of 

 the gas, which seems to have varied consistently thereafter. This sub- 

 ject will be treated in greater detail in subsequent communications. 



We are much indebted to the Rumford Fund of the American Acad- 

 emy for valuable pecuniary assistance in this investigation. 



Summary. 



The desirability of new determinations of the coefficient of expansion 

 by heat under constant pressure for various gases is pointed out. An ap- 

 paratus is described which possesses the advantages of having all the gas 

 at known temperature, of affording the means of measuring pressure 

 to within one-hundredth of a millimeter of mercury, of using a small 

 temperature-interval, and of eliminating many possible constant errors by 

 making the observations of pressure always under similar conditions. 



A table of results of experiments on the expansion of hydrogen under 

 constant pressure, as compared to its previously determined tension in 

 constant volume, shows the possibility of attaining an accuracy equal 

 to that desired, namely, one part in thirty-six hundred. Two other sets 

 of measurements are appended which show that nitrogen expands at 

 essentially the same rate as hydrogen, while carbon dioxide exhibits 

 a very great deviation. It is proposed to make a large number of 

 experiments on different gases at different pressores with the aid of 

 this apparatus. 



It may also be worthy of note that for the first time sodic sulphate has 

 been used on a large scale for maintaining a constant temperature. 



