664 THE PEINCIPLES OF SCIENCE. [chap 



the same ratios as the atomic weights ; now, as nitrogen 

 under such circumstances weighs i4'o6 times as much as 

 hydrogen, we may infer that the atomic weight of nitrogen 

 is about 14.-06, or more probably I4'00, that of hydrogen 

 being unity. There is much evidence, again, that the 

 specific heats of elements are inversely as their atomic 

 weiglits, so that these two classes of quantitative data 

 throw light mutually upon each other. In fact the atomic 

 weight, the atomic volume, and the atomic heat of an 

 element, are quantities so closely connected that the deter- 

 mination of one will lead to that of the others, The 

 chemist has to solve a complicated problem in deciding in 

 the case of each of 60 or 70 elements which mode of deter- 

 mination is most accurate. Modern chemistry presents us 

 with an almost infinitely extensive web of numerical ratios 

 developed out of a few fundamental ratios. 



In hygrometry we have a choice among at least four 

 modes of measuring the quantity of aqueous vapour con- 

 tained in a given bulk of air. We can extract the vapour 

 by absorption in sulphuric acid, and directly weigh its 

 amount ; we can place the air in a barometer tube and 

 observe how much the absorption of the vapour alters 

 the elastic force of the air; we can observe the dew-point 

 of the air, that is the temperature at which the vapour 

 becomes saturated ; or, lastly, we can insert a dry and wet 

 bulb thermometer and observe the temperature of an 

 evaporating surface. The results of each mode can be con- 

 nected by theory with those of the other modes, and we 

 can select for each experiment that mode which is most 

 accurate or most convenient. The chemical method of 

 direct measurement is capable of the greatest accuracy, but 

 is troublesome ; the dry and wet bulb thermometer is 

 sufficiently exact for meteorological purposes and is most 

 easy to use. 



Agreement of Distinct Modes of Measurement. 



Many illustrations might be given of the accordance 

 which has been found to exist in some cases between the 

 results of entirely different methods of arriving at the 

 measurement of a physical quantity. While such accord- 

 ance must, in the absence of information to the contrary, 



