486 Royal Society : — 



the amplitude through any other cable at any other speed could im- 

 mediately be taken from the curve, now verified by experiment. 



Unfortunately this one fact is wanting. The author hopes to be 

 able to supply the want in the second part of this paper. 



" On the Thermal Effects of Fluids in Motion."— Part IV. By 

 J. P. Joule, LL.D., F.R.S , and Professor W. Thomson, F.R.S. 



A brief notice of some of the experiments contained in this paper 

 has already appeared in the * Proceedings.' Their object was to 

 ascertain with accuracy the lowering of temperature, in atmospheric 

 air and other gases, which takes place on passing them through a 

 porous plug from a state of high to one of low pressure. Various 

 pressures were employed, with the result (indicated by the authors in 

 their Part II.) that the thermal effect is approximately proportional to 

 the difference of pressure on the two sides of the plug. The experi- 

 ments were also tried at various temperatures, ranging from 5° to 

 98° Cent., and have shown that the thermal effect, if one of cooling, 

 is approximately proportional to the inverse square of the absolute 

 temperature. Thus, for example, the refrigeration at the freezing 

 temperature is about twice that at 100° Cent. In the case of hy- 

 drogen, the reverse phenomenon of a rise of temperature on passing 

 through the plug was observed, the rise being doubled in quantity 

 when the temperature of the gas was raised to 100°. This result is 

 conformable with the experiments of Regnault, who found that 

 hydrogen, unlike other gases, has its elasticity increased more rapidly 

 than in the inverse ratio of the volume. The authors have also 

 made numerous experiments with mixtures of gases, the remarkable 

 result being that the thermal effect (cooling) of the compound gas 

 is less than it would be if the gases after mixture retained in integrity 

 the physical characters they possessed while in a pure state. 



" On the Spectra of Electric Light, as modified by the Nature of 

 the Electrodes and the Media of Discharge." By the Rev. T. R. 

 Robinson, D.D., F.R.S 



The author, after referring briefly to the researches of previous 

 inquirers, and the hypothesis now generally adopted, that the bright 

 lines observed in these spectra depend so absolutely on the chemical 

 nature of the substances present that their occurrence is an unerring 

 test of that presence, expresses his belief that it cannot be admitted 

 in its full extent without much more decisive proof than has yet been 

 afforded. It assumes, 



1 . That each substance has a set of lines peculiar to itself. 



2. That those lines are not produced or modified by any molecular 

 agent except heat. 



3. That the spectrum of one substance is in nowise modified by 

 the presence of another ; and in such cases both spectra coexist in- 

 dependently, and are merely superposed. 



4. As may be inferred from 2, that electricity does not make 

 matter luminous directly, but only by heating it ; so that the electric 

 spectrum differs in nothing from that produced by heat of sufficient 

 ntensity. 



