Mr. W. J. Macquorn Eankine on the Science of Energetics. 387 



which they contain ; masses of substances of different kinds are compared 

 by means to be afterwards referred to. 



Accident. 



The term " accident'^ will be applied to every variable state of sub- 

 stances, whether consisting in a condition of each part of a substance, 

 how small soever, (which may be called an absolute accident), or in a 

 physical relation between parts of substances, (which may be called a 

 relative accident). Accidents to be the subject of scientific inquiry, must 

 be capable of being measured and expressed by means of quantities. 

 The quantity, even of an absolute accident, can only be expressed by 

 means of a mentally-conceived relation. 



The whole condition or state of a substance, so far as it is variable, is 

 a complex accident ; the independent quantities which are at once neces- 

 sary and sufficient to express completely this complex accident, are inde- 

 pendent accidents. To express the same complex accident, different 

 systems of independent accidents may be employed ; but the number of 

 independent accidents in each system will be the same. 



Examples. — The variable thermic condition of an elastic fluid is a C07n- 

 plex accident, capable of being completely expressed by two independent 

 accidents, which may be any two out of these three quantities — the tem- 

 perature, the density, the piressure—OT any two independent functions of 

 these quantities. 



The condition of strain at a point in an elastic solid, is a complex acci- 

 derU, capable of being completely expressed by six indgjendent accidents, 

 which may be the three elongations of the dimensions, and the three 

 distortions of the faces of a molecule originally cubical, or the lengths 

 and directions of the axes of the ellipsoidal figure assumed by a molecule 

 originally spherical ; or any six independent functions of either of those 

 systems of quantities. 



The distinction of accidents into absolute and relative is to a certain 

 extent arbitrary ; thus, the figure and dimensions of a molecule may be 

 regarded as absolute accidents, when it is considered as a whole, or as 

 relative accidents, when it is considered as made up of parts. Most kinds 

 of accidents are necessarily relative, but some kinds can only be consi- 

 dered as relative accidents when some hypothesis is adopted as to the 

 occult condition of the substances which tliey affect, as when heat is 

 ascribed hypothetically to molecular motions ; and such suppositions are 

 excluded from the present inquiry. 



Accidents may be said to be homogeneous when the quantities express- 

 ing them are capable of being put togetlier, so that the result of the com- 

 bination of the different accidents shall be expressed by one quantity. 

 The number of heterogeneous kinds of accidents is evidently indefinite. 



