44:8 Prof. Tyndall on the Action of Free Molecules on 



lowering of the thermometer ? He answers thus : — " From 

 this it is not to be inferred that the gases do not conduct 

 heat, but merely that in their case conduction is so feeble 

 as to be neutralized by adiathermancy." These are the only 

 words in the note which have any reference to radiation. 



In his next inquiry Magnus dealt directly with the subject 

 of diathermancy, a Preliminary Xote of the investigation 

 being published in the Monatsbericht for February 7th, 1861. 

 This note, like its predecessor, consisted of general and 

 descriptive statements, no actual measurements being given. 

 The completed memoir was first published in PoggendorfFs 

 Annalen for April 1861. For the purposes of this new inquiry 

 the apparatus used in the experiments on gaseous conduction 

 was modified, as shown in fig. 2. To the recipient A B a 

 second one G F was attached, the two being connected by the 

 tubulure shown in the figure. The recipient G F rested upon 

 the plate of an air-pump, on which also stood the thermopile 

 p, with one of its faces turned towards the source. From the 

 pile, through the air-pump plate, wires passed to the galvano- 

 meter. With this apparatus, the absorption by atmospheric 

 air and by oxygen was found to be 11' 12, and by hydrogen 

 14-1 per cent, of the total radiation. The alleged con- 

 ductivity of hydrogen did not therefore manifest itself in 

 these experiments. 



Let us analyze these results. In the first experiments the 

 distance of the thermometer from the source of heat was 

 35 millimetres. The action on the thermometer through a 

 vacuum being represented by 100, the action through air and 

 through oxygen of this depth was found to be 82. The loss 

 of 18 per cent, in air and in oxygen was alleged to be due to 

 the adiathermancy of these media; to which percentage, if we 

 wish to ascertain the total absorption by air, we should have 

 to add such heat as reached the thermometer by conduction. 



Turning now to the modified apparatus, which is evidently 

 drawn to scale, the gas here traversed by the radiant heat 

 was about 275 millimetres in depth, while the stratum tra- 

 versed in the first experiments was, as stated, only 35 milli- 

 metres. Yet in these first experiments an absorption of 18 

 per cent., while in the later ones an absorption of only 11-21 

 per cent., is assigned to air. In other words, when the depth 

 of the aerial stratum was augmented more than sevenfold, 

 the absorption, instead of increasing, fell to less than two 

 thirds of that of the shallower stratum. It is pretty obvious 

 that an influence different from pure absorption came here 

 into play. That influence was convection. 



Anxious to probe this matter to the bottom, and to abolish, 



