C. GENERAL PHYSICS. 155 



sion is positive, but diminishes with increasing temperature. 

 Fourth, in the case of strongly extended caoutchouc, the 

 temperature at which its density is a minimum is lower than 

 ordinary temperatures ; its co-efficient of expansion is there- 

 fore negative at the latter temperature, and increases nu- 

 merically with the temperature. 19 (7, VIII., 146. 



OX THE MOLECULAR HEATS OF SIMILAR COMPOUNDS. 



Professor F. "W. Clarke states that as the result of an ex- 

 tensive comparison between the molecular heats of similar 

 compounds, he finds that these have equal values, not at the 

 same temperature, but at what are called corresponding tem- 

 peratures, which are at equal or nearly equal distances from 

 the respective melting points. Bull. Phil. Soc. Washington, 

 June, 1874. 



OX THE REPULSION DUE TO HEAT. 



In his reply to the criticism of Professor Reynolds, Pro- 

 fessor Crookes states that abundant observations which have 

 been accumulated by him during some years appear in every 

 way to contradict the theory that the phenomena observed 

 by him are due either to air-currents existing within vacuum 

 tubes or to electrical phenomena. As to the theory of Pro- 

 fessor Reynolds, that the effects are the results of evapora- 

 tion and condensation, he satisfactorily shows that while 

 this explanation might sometimes be admissible, yet in gen- 

 eral it requires the adoption of assumptions that seem to be 

 wholly at variance with the facts. He concludes by stating 

 his belief that the repulsion observed by him as accompany- 

 ing the radiation of heat and light is directly due to the 

 impact of the waves upon the surface of the moving mass, 

 and is not a secondary effect through the intervention of air- 

 currents, or electricity, condensation, etc. Whether the sethe- 

 real waves actually strike the object moved, or whether at 

 the boundary of the surface, solid or gaseous, there are in- 

 termediate layers of condensed gas which, taking up the 

 blow, pass it on to the layer beneath, are problems the solu- 

 tion of which must be left to further research ; and, without 

 insisting upon any theory of his own, he proposes it merely 

 as a useful working hypothesis. Any theory will account 

 for some facts, but only the true explanation will satisfy all 



