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through the ether more sluggish, and tend to bring the periods of 

 oscillation into isochronism with the slow undulations of obscure heat, 

 thus enabling the molecules to absorb more effectually such rays as 

 have been made use of in his experiments. He concluded, however, 

 that an agreement in period alone is not sufficient to cause absorp- 

 tion and radiation ; but, in addition to this, the molecules must be 

 so constituted as to furnish points d'appui to the ether. He remarks 

 that the heat of contact is accepted with extreme freedom by rock 

 salt, but a plate of this substance once heated requires a great length 

 of time to cool. This effect is explained by the experiments of Balfour 

 Stewart, which prove that the radiative power of heated rock-salt is 

 extremely feeble. Periodicity, Tyndall remarks, can have no influence 

 here, for the ether is capable of accepting and transmitting impulses 

 of all periods, and the fact that rock-salt requires more time to cool 

 than alum simply proves that the molecules of the former glide 

 through the ether with comparatively smaller resistance, and thus 

 continue moving for a longer time ; while those of the latter, being 

 extremely complex in comparison with rock-salt, present broad sides 

 to the ether and speedily communicate to it the motion which mani- 

 fests itself as heat. This power of gliding through still ether, 

 possessed by the rock-salt molecules must, of course, enable the 

 moving ether to glide round them, and no coincidence of period 

 could, he thought, make such a body a powerful absorber. 



Tyndall extended these experiments to the effect of odours on the 

 absorption of radiant heat. He experimented upon the perfumes 

 arising from patchouli, sandal wood, geranium, oil of cloves, otto of 

 roses, bergamot, lavender, lemon, nearoli, portugal, thyme, rosemary, 

 oil of laurel, cassia, camomile flowers, spikenard, aniseed, and, 

 lastly, musk. Calling the absorptive power of the mixed nitrogen 

 and oxygen of atmospheric air unity, the smallest absoi-ption, 

 namely, that of patchouli, was found to be 30 ; whilst the odour of 

 cassia was 109, and that of aniseed 372. The most surprising result, 

 however, was obtained with musk. It is well known that this sub- 

 stance goes on emitting its odour for months or even years without 

 any perceptible loss of weight ; yet, when it was placed in a small 

 glass tube, and dry air was passed over it in the experimental tube, 

 the inconceivably small amount of this odour gave an absorption 

 expressed in one experiment by 74 and in a second by 72. Several 

 kinds of tea treated in the same manner produced absorptions which 

 varied between 20 and 28. 



Ozone was also submitted to the same test, and was found to have 

 an enormously greater absorptive power than ordinary oxygen. 

 Electrolytic oxygen, which could only contain a small percentage of 

 ozone, was found to have 126 times the absorptive power of ordinary 

 oxygen. This result is extremely surprising in view of the univer- 



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