GROVE'S CORBEL ATION OF PHYSICAL FORCES. 403 



in the transformation of energy even in persons not exclusively devoted to science, 

 but they were long unknown in this country, and produced little direct effect, 

 even in Germany, at the time of their publication. 



The rapid development of thermodynamics, and of other applications of the 

 principle of the conservation of energy, at the beginning of the second half of 

 this century, belongs to a later stage of the history of science than that with 

 which we have to do. 



To form a just estimate of the value of Sir W. Grove's work we must 

 regard it as the instrument by which certain scientific ideas were diffused over 

 a large area, in language sufficiently appropriate to prevent misapprehension, and 

 yet sufficiently familiar to be listened to by persons who would recoil with horror 

 from any statement in which literary convention is sacrificed to precision. 



It is worth while, however, to take note of the progress of evolution by 

 which the words of ordinary language are gradually becoming differentiated and 

 rendered scientifically precise. The fathers of dynamical science found a number 

 of words in common use expressive of action and the results of action, such as 

 force, power, action, impulse, impetus, stress, strain, work, energy, &c. They also 

 had in their minds a number of ideas to be expressed, and they appropriated 

 these words as they best could to express these ideas. But the equivalent 

 words Force Vis, Kraft, came most easily to hand, so that we find them com- 

 pelled to carry almost all the ideas above mentioned, while the other words 

 which might have borne a portion of the load were long left out of scientific 

 language, and retained only their more or less vague meanings as ordinary words. 



Thus we have the expressions Vis acceleratrix, Vis motrix, Vis viva, Vis' 

 mortua, and even Vis inertia, in every one of which, except the second and 

 fourth, the word Vis is used in a sense radically different from that in which 

 it is used in the other expressions. 



Confusion may perhaps be avoided in scientific works when read by scientific 

 students, by means of a careful appropriation of epithets such as those which 

 distinguish the meanings of the word Vis, but as soon as science becomes popu- 

 larised, unless its nomenclature is reformed and arranged upon a better principle, 

 the ideas of popular science will be more confused than those of so-called popular 

 ignorance. 



Thus the " Physical Forces," whose correlation is discussed in the essay 

 before us, are Motion, Heat, Electricity, Light, Magnetism, Chemical Affinity, and 

 " other modes of force." According to the definition of force, as it has been 



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