OF ARTS AND SCIENCES. 57 



27° C. — Having thus eliminated the effect of temperature, we can (after 

 taking a few weighings under as great a variation of temperature and 

 pressure as we can command) easily find the difference of weight 

 which corresponds to a variation of ^ of an inch in the barometer, 

 and we thus obtain a constant for the vessel (or other object weighed) 

 by means of which we can rapidly reduce the weights obtained to the 

 standard of thirty inches' barometric pressure, having previously re- 

 duced them to the standard of 27'' C. for temperature. The weights, 

 having now been corrected for buoyancy, can be compared ; and, 

 although the standards assumed may be as unusual in their associa- 

 tion as is one of them in its value, they are as legitimate as any others, 

 and will be found in practice more convenient. 



To apply this method of reduction we simply leave the load equi- 

 poised on the balance, shifting the rider with the varying weight, and 

 noting the corresponding temperatures and pressures, until a sufficient 

 difference has been observed ; and a difference corresponding to 20° C, 

 or two inches of mercury, is adequate in most cases. The process 

 corresponds to calibrating a flask, and the constant, once obtained, can 

 be afterwards used for the same vessel, unless the weight of its con- 

 tents is materially altered. The following examples will show the 

 application of the method. 



In each case the load was a closed absorption tube of peculiarly 

 irregular construction, but not much larger in volume than those 

 generally used in organic analysis. We give in the accompanying 

 tables, first, the date ; secondly, the observed weight ; thirdly, the 

 temperature of the balance-case ; and, fourthly, the height of the 

 barometer at the time of weighing in tenths of an inch. These are 

 the observed data. In the fifth column we give the reduced heights 

 of the barometer for 27° C, and these values are obtained by simply 

 subtracting the observed temperatures from 27°, and adding the re- 

 mainders to the observed barometric heights. Below the table we 

 print in each case the largest weight observed over the smallest 

 weight observed, and on the same lines the corresponding reduced 

 barometric heights. Dividing now the difference of weight in milli- 

 grammes by the difference of height in tenths of an inch, we obtain 

 the value last given, which we have called the*" constant." "With this 

 constant we can very rapidly reduce all the weights to the common 

 standard of thirty inches, and this we do by multiplying the difference 

 between 300 and the reduced barometric heights by this constant, and 

 adding or subtracting the product, as the case may be, to or from the 

 observed weights. 



