228 LIFE AND LABORS OF HENRY GUSTAVUS MAGNUS. 



sources of beat employed. The most marked differences are obtained 

 from a source of black beat at 100° C. As to the vapor of water con- 

 tained in the air Magnus observed that it was appreciably absorbent 

 only when in the condition of a transparent gas. This subject gave rise 

 to a long controversy between Magnus and Mr. Tyndall. 



The English savant had undertaken the study of this interesting sub- 

 ject and gave the results of his investigations in a discourse before the 

 Eoyal Society only a few days before Magnus communicated his to the 

 Academy of Berlin. The two savants agreed upon all points, except 

 upon the transmission of radiant heat through dam j) air. Contrary to 

 Magnus, Mr. Tyndal found that the vapor of water contained in the air 

 absorbed radiant heat forty, fifty, and even sixty times more than the 

 air itself; and yet this result was obtained, not with air absolutely 

 saturated with moisture, but with ordinary open air or with that of his 

 laboratory. The want of accord between the two experimenters in this 

 particular remains, in spite of repeated efforts to discover the cause in their 

 different modes of operating. Magnus accounted for the enormous ab- 

 sorption obtained by Mr. Tyndidl by the condensation of the vapor upon 

 the interior surface of the tube used, or upon the plates of rock-salt 

 which closed the extremity of this tube. The English savant repeated 

 his experiments with the greatest care, but always with the same I'csult. 

 The error, on whichever side it exists, remains undiscovered, and some 

 idea may be formed of the difficulty of this qucstiou when two such 

 skillful experimenters fail to solve it. 



Magnus made other interesting experiments upon radiant heat, espe- 

 cially upon the variation in the emissive power of a body with the 

 degree of polish of its surface, and he vshowed that the increase of this 

 emissive power in an unpolished body is greater, not because the super- 

 ficial stratum is less dense, but on account of its discontinuity. He 

 also observed that the increase in the emissive power of platina was 

 confined to the radiations of red or nearly red heat. He discovered, also, 

 the property which sylviue or chloride of potassium possesses, in com- 

 mon with rock-salt, of being almost entirely diathermal and of trans- 

 mitting equally heat jH^oceeding from very different sources. It was 

 interesting to find a new analogy between these two substances, which 

 in many of their properties, as well as in their chemical composition, 

 are completely identical. In respect to the remarkable diathermality of 

 rock-salt, we shall see how entirely he modified the theory of JMelloui, 

 which had been generally adopted. 



Shortly before his death Magnus published a memoir upon the emis- 

 sion, absorption, and reflection of heat by bodies at low temperatures. 

 This article, given entire in the Archives, shows that different bodies 

 emit, absorb, and reflect, at temperatures in the neighborhood of 100° 

 C, very different calorific radiations; so that were our eyes capable of 

 perceiving the radiations of black heat, the bodies ex])osed to them 

 would appeal' to us of very different colors, as is the case when they are 



