Dr. W. J. M. Rankine on Thermodynamics. 103 



spectrum C I. can be obtained both from flame and by the use of 

 the electric spark. 



The sodium-lines /3, 7, 8, and s are seen in the spectrum of the 

 electric light, of the spark of an induction-coil between sodium 

 poles in air, both with and without the Ley den jar, and are ob- 

 tained simultaneously with the hydrogen-lines a, /3, and 7 in a 

 hydrogen vacuum-tube. The simplest mode of obtaining them 

 is to heat the narrow part of the vacuum-tube with a Bunsen 

 flame; the discharge inside the hot part of the tube becomes 

 yellow and exhibits the sodium-lines brilliantly. 



I have confirmed the results obtained by Wiillner in his expe- 

 riments on hydrogen under pressure, and have pushed the pres- 

 sure to nine atmospheres. The spectrum of the spark of an in- 

 duction-coil (without condenser) in the gas at nine atmospheres' 

 pressure is still far from being continuous. H a is still a very 

 bright and distinct line ; H /3 and y are merely maxima of light. 



XIII. On Thermodynamics. By W. J. Macquorn Rankine, 

 C.E., LL.D., F.R.SS.L. $ E., $<?. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IF I rightly understand the paper of the Rev. J. M. Heath, 

 published in the Philosophical Magazine for July 1870, 

 p. 51, he lays down a principle which may be virtually expressed 

 by saying that the work done by a force in overcoming attrac- 

 tions or repulsions cannot take effect in producing heat, — that is, 

 in accelerating molecular motions. That principle is perfectly 

 correct, and is an obvious consequence of the laws of motion ; 

 and every one who knows those laws must agree with Mr. Heath 

 when he states it. But from the remarks with which his state- 

 ment is accompanied, he seems not to be aware that this very 

 principle has been most carefully kept in view by every author 

 of original researches in thermodynamics, and by every writer on 

 the subject who has understood those researches. In fact the 

 problem which is solved by the general equation of thermody- 

 namics may be stated as follows: — A certain quantity of work 

 being done by the action of external forces on a body in a cer- 

 tain way, to distinguish that quantity of work into two parts, 

 one of which is expended in overcoming molecular attractions 

 and repulsions, and the other in accelerating molecular motions. 

 A system of particles contained within a vessel and in, a state 

 of rest, being kept in cequilibrio by their mutual attractions and 

 repulsions, exerts a pressure or a tension against the internal sur- 

 face of that vessel according as repulsions or attractions predo- 



