92 Prof. Tyndall on Sounding and Sensitive Flames. 



is found experimentally to be always the same when those 

 masses are compared by means of the velocities impressed on 

 them by different forces and in different times, and is also the 

 same whether each of the masses is measured as a whole or as 

 the sum of a set of parts. 



Assuming those definitions as merely verbal, without refer- 

 ence to their reality, the laws of motion take the form of verbal 

 truisms ; but when experiment and observation inform us that 

 permanent relations exist among real bodies and real events 

 corresponding to the definitions, those apparent truisms become 

 statements of fact. 



16. One of the chief objects of mathematical physics is to 

 ascertain, by the help of experiment and observation, what phy- 

 sical quantities or functions are " conserved. - " Such quantities 

 or functions are, for example, 



I. The mass of every particle of matter — conserved at all 

 times and under all circumstances. 



II. The resultant momentum of a body or system of bodies 

 — conserved so long as internal forces act alone. 



III. The resultant angular momentum of a body or system of 

 bodies — conserved so long as internal forces act alone. 



IV. The total energy of a body or system of bodies — con- 

 served so long as internal forces act alone. 



V. The thermodynamic function — conserved in a body while 

 it neither receives nor gives out heat. 



In defining such physical quantities as those, it is almost, if 

 not quite, impossible to avoid making the definition imply the 

 property of conservation ; so that when the fact of conservation 

 is stated, it has the form of a truism. 



17. In conclusion, it appears to me that the making of a phy- 

 sical law wear the appearance of a truism, so far from being a 

 ground of objection to the definition of a physical term, is rather 

 a proof that such definition has been framed in strict accordance 

 with reality. 



Glasgow University, 

 January 9, 1867. 



XIV. On Sounding and Sensitive Flames. By John Tyndall, 

 LL.D.j F.R.S., Professor of Natural Philosophy R.I., #<?.* 



THE sounding of a hydrogen -flame when enclosed within a 

 glass tube was, I believe, first noticed by Dr. Higgins in 

 1777. The subject has been since investigated by Chladni, De 

 la Rive, Faraday, Wheatstone, Rijke, Sondhauss, and Kundt. The 

 action of unisonant sounds on flames enclosed in tubes has been 



* Abstract of a lecture delivered at the Royal Institution of Great Britain, 

 on Friday, January 18, 1867. 



