336 THE PRINCIPLES OF SCIENCE. [CHAP. 



way analysed and separately measured before we possess 

 the materials for inductive treatment. 



Illustrations of the Complication of Effects. 



It is easy to bring forward a multitude of instances to 

 show that a phenomenon is seldom to be observed simple 

 and alone. A more or less elaborate process of analysis 

 is almost always necessary. Thus if an experimentalist 

 wishes to observe and measure the expansion of a liquid 

 by heat, he places it in a thermometer tube and registers 

 the rise of the column of liquid in the narrow tube. But 

 he cannot heat the liquid without also heating the glass, 

 so that the change observed is really the difference between 

 the expansions of the liquid and the glass. More minute 

 investigation will show the necessity perhaps of allowing 

 for further minute effects, namely the compression of the 

 liquid and the expansion of the bulb due to the increased 

 pressure of the column as it becomes lengthened. 



In a great many cases an observed effect will be 

 apparently at least the simple sum of two separate and 

 independent effects. The heat evolved in the combustion 

 of oil is partly due to the carbon and partly to the 

 hydrogen. A measurement of the heat yielded by the two 

 jointly, cannot inform us how much proceeds from the 

 one and how much from the other. If by some separate 

 determination we can ascertain how much the hydrogen 

 yields, then by mere subtraction we learn what is due 

 to the carbon; and vice versa. The heat conveyed by a 

 liquid, may be partly conveyed by true conduction, partly 

 by convection. The light dispersed in the interior of a 

 liquid consists both of what is reflected by floating 

 particles and what is due to true fluorescence; 1 and we 

 must find some mode of determining one portion before 

 we can learn the other. The apparent motion of the spots 

 on the sun, is the algebraic sum of the sun's axial 

 rotation, and of the proper motion of the spots upon the 

 sun's surface; hence the difficulty of ascertaining by 

 direct observations the period of the sun's rotation. 



We cannot obtain the weight of a portion of liquid 



1 Stokes, Philosophical Transactions (1852), vol. cxlii. p. 529. 



