94 PEOFESSOE TYNDALL ON THE INFLUENCE OF COLOUE 



this rule exhibited in Table III. are marked with asterisks. This result, I think, is 

 what might fairly be expected ; for the character which enables a molecule of one sub- 

 stance to radiate a greater quantity of heat than another, may also be expected to influence 

 its rate of oscillation. Hence, as a general rule, a greater dissonance will exist between 

 the vibrating periods of good radiators and bad radiators, than between the periods of 

 the members of either class. But the greater the dissonance the less will be the absorp- 

 tion ; hence, as regards transmission through rock-salt, we have reason to expect that 

 powerful radiators will find a more open door to their emission than feeble ones. This 

 is, as I have said, in general the case. But the rule is not without its exceptions, and the 

 most striking of these is the case of black platinum, which, though but a moderate 

 radiator, sends a greater propoi'tion of heat through rock-salt than any other known 

 substance. 



In his latest investigation Knoblauch examined at great length the diathermancy of 

 rock-salt. With his usual acuteness he points out several possible sources of error, and 

 with his customary skill he neutralizes these sources. His conclusion is the same as that 

 of Melloni, namely, that rock-salt transmits in the same proportion all sorts of rays. 

 On the opposite side we find the experiments of MM, De la Provostate and Desains, 

 and those of Mr. Balfoue Stewart*, both of which are discussed by Knoblauch. He 

 differs from those experimenters, while my results bear them out. Considering the slow 

 augmentation of transmission which the foregoing Tables reveal, and the considerable 

 number of bodies whose heat is transmitted in almost the same proportion by rock-salt, 

 it is easy to see that, where the number of radiants is restricted, such a uniformity 

 of transmission might manifest itself as would lead to the conclusion of Melloni and 

 Knoblauch. It was only by the selection and extension of the substances chosen as 

 radiators that the difierences were brought out with the distinctness recorded in the 

 foregoing Tables. 



The differences in point of quality and the absence of perfect diathermancy in rock- 

 salt appear more striking when instead of the transmissions we take the absorptions. In 

 the case of the radiation from powdered rock-salt, for example, 37 '2 per cent, of the 

 whole radiation is intercepted by the rock-salt plate. According to Melloni, between 

 7 and 8 per cent, of this is lost by refiexion at the two surfaces of the salt. This would 

 leave in round numbers a true absorption of 30 per cent, by the plate of rock-salt. In 

 the case of black platinum, the absorption similarly deduced amounts to only 4 per cent, 

 of the total radiation. Instead, therefore, of the radiation from those two sources being 

 absorbed in the same proportion, the ratio in the one case is more than seven times that 

 in the other. For the sake of illustration here follow a few of the absorptions deter- 

 mined in this way : — 



• I think the important experiment first executed by Mr. Balfoub Stewabt, of rock-salt radiating through 

 rock-salt, is by itself sufficient to demonstrate in the most unequivocal manner that this substance is not equally 

 pervious to all kinds of rays. 



