338 Prof. Clausius on the Application of the Mechanical 



pletely imitated by any combination ; that the theory of three 

 primitive colours must be abandoned, the colours produced by 

 the combinations of any three being not so pure and vivid as 

 those of the spectrum; and that white can be obtained in an, 

 infinite number of ways without the eye being able to distinguish 

 one white from another. In fact, the number and variety of the 

 explanations of this class of phaenomena given by the undulatory 

 theory may be adduced as a confirmation of its truth. No other 

 theory of hght has made the least approach to such explanations. 



Cambridge Observatory, 

 October 10, 1856. 



XLII. On the Application of the Mechanical Theory of Heat 

 to the Steam-engine. By R. Clausius. 

 [Continued from p. 266.] 

 27. ^T^HE influence exercised by the difference of pressure in 

 -I- the boiler and cylinder upon the work performed has 

 hitherto been most completely treated of by Pambour in his work 

 on the Thiorie des Machines a Vapeur, Before entering upon the 

 subject myself, therefore, I may be allowed to state the most 

 essential parts of his treatment, altering only the notation, and 

 neglecting the magnitudes which have reference to friction. By 

 this means it will be easier, on the one hand, to judge how far 

 this treatment is no longer in accordance with our more recent 

 knowledge of heat, and, on the other, to add to it the new 

 method of treatment which, in my opinion, must be substituted 

 for the former one. 



28. The two laws which, as was before mentioned, were for- 

 merly very generally applied to steam, form the basis of Pam- 

 bour's theory. The first of these is Wattes law, according to 

 which the sum of the latent and sensible heat is constant. 

 Prom this law it was concluded, that when a quantity of steam 

 at its maximum density is enclosed within a surface impenetrable 

 to heat, and the volume of the enclosing space is either increased 

 or diminished, the steam will neither become over-heated nor 

 partially condensed, but will remain precisely at its maximum 

 density ; and it was further assumed that this would take place 

 quite independently of the manner in which the change of volume 

 occurred, whether thereby the steam had, or had not, to overcome 

 a pressure corresponding to its own expansive force. Pambour 

 supposed that the steam in the cylinder of a steam-engine de- 

 ported itself thus ; and at the same time he did not assume that 

 the particles of water, which in this case are mixed with the 

 steam, could exert any appreciable influence. 



Further, in order to establish a more accurate relation between 



